Hilchos Birchas Hamazon
 
526. Upon completing a meal one should not remove the tablecloth and the leftover bread/challah until after Birchas Hamazon, and one who does not leave bread on the table will never see "Siman Bracha". Shulchan Aruch w/Mishnah Brurah 180:1,2
 
527. Three reasons are mentioned; a) It should be clear that one is bentching to Hashem for his great good and kindness in providing sustenance to all his creations, and
b) The berachos (for our continued sustenance and parnassah) in Birchas Hamazon can not be effective upon an "empty object" (ie. and empty table), and
c) One should always have something left on the table in case a poor person happens by. Shulchan Aruch w/Mishnah Brurah 180:1,2
 
528. One who leaves bread on the table until after Birchas Hamozon (thereby "lengthening his table" for the poor) will be blessed with long life (measure for measure).
Shulchan Aruch w/Mishnah Brurah 180:2
 
529. If upon completing the meal (prior to Birchas Hamozon) one sees that all the bread has been eaten, one may bring a new complete loaf to the table for Birchas Hamozon (for the reasons cited in Halacha #527). Shulchan Aruch w/Mishnah Brurah 180:2
 
530. Prior to washing Mayim Achronim, if one has the custom to wash at the table, one should remove any crumbs from the table to be sure that the crumbs do not get wet and become messy on the table. The larger piece(s) of bread that is to remain on the table until after bentching should be set in a place where it is unlikely to become wet.
Shulchan Aruch w/Mishnah Brurah 180:3
 
531. If, on the other hand, one has the custom to wash Mayim Achronim away from the table (e.g. in a sink) one need not clear the crumbs from the table before bentching.
Shulchan Aruch w/Mishnah Brurah 180:3
 
532. If one steps on bread crumbs (even small (aggregate) amounts of less than a kezayis), it can lead to poverty. Therefore, one should be careful to dispose of the crumbs by either wrapping them and then discarding them in the garbage, or throwing them into water.
Shulchan Aruch w/Mishnah Brurah 180:4
 
533. Before beginning Birchas Hamozon one should remove, or cover the knives because;
a) Metal (i.e. a knife), which shortens life, should not be left on a table which is compared to a Mizbeach (alter), which lengthens life (through the offerings brought), and
b) It once happened that someone was so overcome about the destruction of the Bais Hamikdash upon reaching "Boneh Yerushalayim" that he grabbed a knife and stabbed himself. Therefore Chazal decreed that knives should be removed or covered before bentching.
Shulchan Aruch w/Mishnah Brurah 180:5
 
534. Washing one's hands before Birchas Hamazon (known as Mayim Achronim) is an obligation. Although some people do not have this custom the Mishnah Brurah cites four eminent Poskim; The Vilna Gaon, The Magen Avraham, The Maharshal and The Birkei Yosef; as being of the opinion that one should be strict about performing this custom.
Shulchan Aruch w/Mishnah Brurah 181:1,10
 
535. Two reasons are brought for this minhag; a) One's hands become dirty while eating and are ineligible to make a beracha; and b) To clean hands of certain types of salt that can cause blindness if touched to the eyes. Shulchan Aruch w/Mishnah Brurah 181:1
 
536.  If one did not have anything with which to wash Mayim Achronim and already bentched, one should was his hands as soon as he has access to water and is able to wash.
Shulchan Aruch w/Mishnah Brurah 181:2
 
537. If one only has enough water for washing before bread or for Mayim Achronim, one should use the water to wash before eating bread.
Shulchan Aruch w/Mishnah Brurah 181:2
 
538. When washing Mayim Achronim one should preferably have the water fall into a bowl, and not onto the floor because it is a danger for someone who may pass over the used water (which has a "ruach ra'ah"). Shulchan Aruch w/Mishnah Brurah 181:2
 
539. However, if one does not have a bowl or other kli, one may wash under the table, so that the water falls onto the floor at a place where people do not walk.
Shulchan Aruch w/Mishnah Brurah 181:2
 
540. L'chatchila one should wash Mayim Achronim until the knuckles to fulfill all opinions. Bi'dieved one is yotze if he washed until the 2nd joint of the fingers (and the 1st joint of the thumb) because that is where the food usually touches when eating.
Shulchan Aruch w/Mishnah Brurah 181:4
 
541. The Mishnah Brurah says "It (makes me feel) is bad to see people who are careful with washing Mayim Achronim, but they are not fulfilling the obligation of the law at all because they only put some drops of water on their fingertips which do not even reach the end of the 1st finger joints....". Shulchan Aruch w/Mishnah Brurah  181:4
 
Note: The typical Mayim Achronim cups in use today generally only hold enough water for 1-2 people to properly wash Mayim Achronim. (Even a full cup of water brought to the table will only contain enough water for 4-6 people). Therefore, it may be advisable, especially for families who have more than 2 people eating at the table, to wash at a sink, to insure the proper fulfillment of this Mitzvah.
 
542. L'chatchilah one should wash Mayim Achronim using cold water. Bi'dieved one may use warm water, but one should never use hot water.
Shulchan Aruch w/Mishnah Brurah 181:3
 
543. When washing Mayim Achronim one should face the fingers downwards to allow the water to draw any leftover food and salt off the hands.
Shulchan Aruch w/Mishnah Brurah 181:5
 
544. When washing Mayim Achronim at the table; if there are up to five people at the table the one who will lead the bentching should wash Mayim Achronim first to allow him the few moments while the others are washing to review and prepare the Birchas Hazimun.
Shulchan Aruch w/Mishnah Brurah 181:6
 
545. If more than 5 people are at the table it would be an embarrassment for the one who will lead bentching to wait while everyone else washes, and also it would constitute a hefsek (an interruption). Therefore, the washing should start with the one sitting at the end of the table, continuing along the table until there are only five people left. Then follow the order as stated in #544 above. Shulchan Aruch w/Mishnah Brurah 181:6
 
546. No Beracha is said when washing Mayim Achronim.
Shulchan Aruch w/Mishnah Brurah 181:7
 
547. There is a machlokes amongst the Rishonim whether one should dry his hands after washing Mayim Achronim. Therefore, L'chatchila, one should dry his hands to fulfill the opinion of the Rambam who requires the hands to be dried.
Shulchan Aruch w/Mishnah Brurah 181:8
 
548. If one does not have water available for Mayim Achronim, bidieved, one may use any liquid such as oil, or milk. (One should not use wine because of its importance).
Shulchan Aruch w/Mishnah Brurah 181:9
 
549. Various types of water that would be unacceptable to use for Mayim Rishonim (washing for Hamotzie) would be acceptable for Mayim Achronim, such as; used water, unclear water, water from which an animal drank.
Shulchan Aruch w/Mishnah Brurah 181:9
 
550. One need not use a cup or other kli (vessel) to wash Mayim Achronim. One may simply place the hands under a faucet to fulfill the Mitzvah.
Shulchan Aruch w/Mishnah Brurah 181:9
 
551. Various types of items/objects that would have to be removed from the hands before washing for Mayim Rishonim (washing for Hamotzie) because they are a Chatzitza (separation) between the water and the skin do not invalidate when washing for Mayim Achronim.
Shulchan Aruch w/Mishnah Brurah 181:9
 
552. One need only wash each hand once for Mayim Achronim.
Shulchan Aruch w/Mishnah Brurah 181:9
 
553. Once one has washed Mayim Achronim one may not interrupt with any speech, including for Divrei Torah, before beginning Birchas Hamazon,.
Shulchan Aruch w/Mishnah Brurah 181:10
 
554. Anytime (even during the week) that there is a Mizuman (of three men who ate together), and there is wine available in the home, one should bentch with a cup of wine to "do the Mitzvah in the best way". Shulchan Aruch w/Mishnah Brurah 182:1,2
 
555. The reason to bentch over wine is because it shows honor and praise to Hashem when one arranges his blessing and praise to Hashem with a cup of wine.
Shulchan Aruch w/Mishnah Brurah 182:1
 
[556-622 Hilchos Chodesh Elul, Rosh Hashanah, Yom Kippur, Sukkos]
 
623. A cup of wine from which someone drank even a small amount is called a "Kos Pogum" and is invalid (L'chatchila) to be used for Birchas Hamazon (or Kiddush or Havdalah), but may still otherwise be drunk (after making a beracha of Ha'Gofen).
Shulchan Aruch w/Mishnah Brurah 182:3, 271:10, 296:1
 
624. A cup of wine only becomes invalidated and is called "Kos Pogum" if a person drank directly from the cup. It is not invalidated if some wine was poured out, or if a Jew stuck a finger in and tasted the wine off the finger. Shulchan Aruch w/Mishnah Brurah 182:3
 
625. A "Kos Pogum" can be restored by pouring a little bit of wine or other liquid into the cup and then pouring the entire cup back into the wine bottle (containing at least more wine than that being poured in from the cup). The wine in the bottle is now valid again for use for Birchas Hamazon (or Kiddush or Havdalah). Shulchan Aruch w/Mishnah Brurah 182:5
 
626. One must be careful when pouring wine or other liquid into the "Kos Pogum" to use only wine or liquid which itself is not "Pogum" - it has not been drunk from directly from a cup or bottle. Otherwise, it will not have the ability to correct the invalidity of the wine in the "Kos Pogum".
Shulchan Aruch w/Mishnah Brurah 182:5,6
 
627. Bi'dieved, if one has no other wine to use to correct a "Kos Pogum" one should pour the wine into a smaller cup (that holds at least a revi'is) thereby partially correcting the invalidity of the wine. Shulchan Aruch w/Mishnah Brurah 182:7
 
628. If one has no smaller cup available, one may Bi'dieved use the "Kos Pogum" for Birchas Hamazon (or Kiddush or Havdalah).
Shulchan Aruch w/Mishnah Brurah182:7
 
629. When the one who is leading the Birchas Hamazon (or Kiddush or Havdalah) drinks from the cup of wine it does not make it a "Kos Pogum" for the others at the table and they may drink from the same cup. The only concern is that it not be a "Kos Pogum" for the one who is making the beracha (on the Kiddush or Havdalah) after Birchas Hamazon.
Shulchan Aruch w/Mishnah Brurah 182:4
 
630. If someone drank from a cup of wine using a straw it is not "Pogum", but one should still L'chatchila treat it as a "Kos Pogum", and correct it (as in Halacha #625).
Shulchan Aruch w/Mishnah Brurah 182:7, See Shaarei Tshuva 10, S'U Sh'vus Yaakov 3:14
 
631. A cup that will be used for Birchas Hamazon (or Kiddush or Havdalah) should be rinsed clean before use. However, if the cup has been cleaned well before, it is not necessary to rinse it again. Shulchan Aruch w/Mishnah Brurah 183:1
 
632. L'chatchila the cup (Kos shel Beracha) should be filled up to the top, but B'idieved, as long as the cup has a Revi'is of wine this suffices. (Note: There are differing opinions as to the amount of a Revi'is ranging from 3.0 oz. to 5.3 oz. For Kiddush Friday night most opinions are that one should use a larger shiur which according to R' Moshe Feinstein is 4.42 oz).
Shulchan Aruch w/Mishnah Brurah 183:2, 271 MB42
 
633. L'chatchila the cup used for a Kos Shel Beracha should not be broken, split, cracked (even at the base), or missing even a small amount at the top rim. Bi'dieved, if one has no other cup, one may used a damaged cup. Shulchan Aruch w/Mishnah Brurah 183:3
 
634. The Magen Avraham cites the Bach who says that a Yorei Shomayim (G-d fearing person) should say Birchas Hamazon while wearing a hat. (Some have the minhag to wear a jacket as well). The Mishnah Brurah adds that the minhag is to wear a hat even when one is bentching alone, without a Kos. Shulchan Aruch w/Mishnah Brurah 183:3
 
635. The one who is bentching with the Kos should accept the cup from another person with two hands to show the importance of the Kos, and then grasp the Kos in his right hand (even a left-handed person), with the cup sitting in the palm of the hand and the fingers wrapped around the sides. Shulchan Aruch w/Mishnah Brurah 183:4
 
636. One should not hold a Kos Shel Beracha while wearing gloves.
Shulchan Aruch w/Mishnah Brurah 183:4
 
637. After making the beracha of Hagafen following Birchas Hamazon the wine should be passed to one's wife to drink from (even if she did not eat together with the zimun) because through the Kos Shel Beracha she will be blessed. Shulchan Aruch w/Mishnah Brurah 183:4
 
638. Likewise, if a guest has been honored to lead the Birchas Hamazon he should pass the wine to the host to drink from, so that he will be blessed as well.
Shulchan Aruch w/Mishnah Brurah 183:4
 
639. The one who is leading the Birchas Hamazon should say the words out loud and the others should say the words along with him in a lower tone. L'chatchila the leader should say the whole Birchas Hamazon out loud. B'idieved he must at least say the first beracha out loud for it to be considered a zimun (of 3 or 10, as the case may be). Shulchan Aruch w/Mishnah Brurah 183:7
 
640. Before the end of each beracha the others should finish first in order to be able to answer Amen to the beracha of the leader. Shulchan Aruch w/Mishnah Brurah 183:7
 
641. One may not speak during Birchas Hamazon, just as one may not speak during Sh'mona Esrei. Shulchan Aruch w/Mishnah Brurah 183:8
 
642. One should sit (straight, not in a longing position) when one says Birchas Hamazon, even if one ate while standing. However, B'idieved, if one bentched while standing or even walking, one need not repeat Birchas Hamazon. Shulchan Aruch w/Mishnah Brurah 183:9
 
643. When one says the beracha of "Al Hamichya" after eating Mezonos one should sit, just as one should sit during Birchas Hamazon. Shulchan Aruch w/Mishnah Brurah 183:10
 
644. If one was traveling, and he ate while walking/moving, one need not sit for Birchas Hamazon if the delay caused by sitting for Birchas Hamazon will disturb his concentration on the beracha, and he may bentch while walking/moving.
Shulchan Aruch w/Mishnah Brurah 183:11
 
645. During Birchas Hamazon, (or while davening, or saying any beracha), one may not be simultaneously be doing any other activity (including reading, learning Torah, or even any minor physical activity), because this takes away from the importance of the mitzvah at hand.
Shulchan Aruch w/Mishnah Brurah 183:12, 191:3
 
646. The aforementioned halacha is applicable regardless of whether one was doing the other activity before starting Birchas Hamazon, or not.
Shulchan Aruch w/Mishnah Brurah 183:12, 191:3
 
647. One must bentch (and say Al Hamichya) in the place where one ate. For any minor reason one may bentch anywhere within four Amos (appx. 8 feet) and it is considered one's place for this purpose. Shulchan Aruch w/Mishnah Brurah 184:1, 184:3
 
648. One should not leave the home or building where one ate without first bentching.
Shulchan Aruch w/Mishnah Brurah 184:1
 
649. If one left the place where one ate without bentching (and one does not have any more bread to eat at the 2nd location), L'chatchila one should return to the place he ate and bentch (so long as by waiting to return, the time allowed to bentch after a meal - 72 minutes - will not pass). Shulchan Aruch w/Mishnah Brurah 184:1 MB7
 
650. Bidieved, if one did not return to the place he ate, rather, he bentched where he was, he is Yotze and need not return to the place he ate to repeat the bentching.
Shulchan Aruch w/Mishnah Brurah 184:1 MB5
 
651. If one ate in one place and wishes to continue his meal at a 2nd location, (and had in mind at the time of Hamotzie to move to a 2nd location) one need not bentch before leaving, and one may (L'chatchila) move to the 2nd location (in another house), eat some bread (even less than a k'zayis) and bentch once at the 2nd location. (If one did not have the move in mind at Hamotzie, eating bread at the 2nd location only permits one to avoid returning to the 1st location to bentch B'idieved). Shulchan Aruch w/Mishnah Brurah 184:2, 178:2
 
652. One may only move to a 2nd location and rely on the same beracha Rishona if one has not yet become hungry after eating the first time (generally 72 minutes).
Shulchan Aruch w/Mishnah Brurah 184:2
 
653. If one ate bread (and one is "satisfied") and one is not sure if he bentched, one should bentch (the entire Birchas Hamazon - including the 4th beracha which is M'idirabonon) because the obligation to bentch is min Hatorah (sofek D'oraysah L'chumrah).
Shulchan Aruch w/Mishnah Brurah 184:4
 
654. If one is not "satisfied" the obligation to bentch is only M'idirabonon and one need not bentch m'sofek. However, one who is a Yorei Shomayim should be machmir and wash again, make a new Hamotzie, eat a K'zayis, and bentch.
Shulchan Aruch w/Mishnah Brurah 184:4
 
655. One may bentch up to 72 minutes after eating.
Shulchan Aruch w/Mishnah Brurah 184:5
 
656. One must eat at least a k'zayis (volume of ½ an egg - 1 oz. or 28.35 grams) of bread to be obligated to bentch. If one eats less than a k'zayis, although one may not bentch, one still must make Hamotzie before eating. Shulchan Aruch w/Mishnah Brurah 184:6
 
657. When saying Birchas Hamazon one must hear the words that one says with his mouth. B'idieved if one did not hear the words but did at least say them with his mouth (and did not simply look at the words and think of them); he will receive some S'char (reward) but it is not considered as if he bentched properly. If one did not say the words, one is not Yotze at all.
Shulchan Aruch w/Mishnah Brurah 185:2
 
658. If one is sick, or there is some other circumstance that prevents him from saying the words, one should think the words of bentching. If anytime within 72 minutes after eating he is able to say the words, he should repeat the bentching and say all the words.
Shulchan Aruch w/Mishnah Brurah 185:2
 
659. The Sefer Hachinuch writes; One who is careful with the mitzvah of Birchas Hamazon will always have Parnasah in abundance and in a respectful manner. One who wants to be careful should always bentch while reading the words in a siddur (or bentcher) and not bentch by heart.
Shulchan Aruch w/Mishnah Brurah 185:1, see also Be'er Haitaiv
 
660. The Sefer Hachasidim writes; A person once died and appeared to a relative in a dream and told him that he is being judged every day for not having been careful to say all his berachos with proper kavannah. Shulchan Aruch w/Mishnah Brurah 185:1, see also Be'er Haitaiv
 
661. The Magen Avaraham and Eliyahu Rabbah say that one should always bentch in a loud voice because hearing the words one is saying will help one to have kavannah, and help to make sure that one does not skip R'itzay on Shabbos or Y'aaleh V'yovoi on Rosh Chodesh and Yom Tov. Shulchan Aruch w/Mishnah Brurah 185:3
 
662. If one is so drunk that he is unable to say the words properly (but he can still "speak before a king") one should still bentch. L'chatchila, if one is drinking and feels that he may be getting drunk, one should hurry up and bentch before he reaches a state in which he may not be able to bentch properly to Hashem. Shulchan Aruch w/Mishnah Brurah 185:4,5 MB4,6
 
663. Women are obligated to bentch. There is disagreement amongst the Rishonim whether the obligation is a Torah mandated obligation, or a Rabbinically mandated obligation.
Shulchan Aruch w/Mishnah Brurah 186:1
 
664. A woman who ate "K'dei S'viah" (enough to feel full - which is the shiur Min Hatorah) and is unsure whether or not she bentched may rely on the opinion of the poskim and Rishonim who consider her obligation to bentch to be Biblical (which would require one to be stringent in cases of doubt) and therefore she may bentch.
Shulchan Aruch w/Mishnah Brurah 186:1, see also Biur Halacha
 
665. A child is obligated to bentch Rabbinically for Chinuch - to be properly trained in the mitzvah prior to becoming a Bar Mitzvah (or Bas Mitzvah) even if he only ate a K'zayis (1/2 an egg) of bread. Shulchan Aruch w/Mishnah Brurah 186:2
 
666. At the end of the 3rd beracha one should be careful to pause between "Boneh Yerushalayim" and "Amen" because it should be clear that the Amen is not part of the beracha.
Shulchan Aruch w/Mishnah Brurah 188:2
 
667. Anytime one must say both "Ritzay" (for Shabbos) and "Yaaleh Viyovoi" (for Rosh Chodesh or Yom Tov) in bentching, "Ritzay" is said first because it is said more often.
Shulchan Aruch w/Mishnah Brurah 188:5
 
668. However, if one said "Ya'aleh Viyovoi" first, Bi'dieved one is Yotze and need not repeat either one. Shulchan Aruch w/Mishnah Brurah 188:5
 
669. If one forgot to say "Ritzay" (on Shabbos) or "Ya'aleh Viyovoi" (on Yom Tov, Chol Hamoed, or Rosh Chodesh - both night and day) in Birchas Hamazon, (or if one is unsure if he said it), the following rules apply; Shulchan Aruch w/Mishnah Brurah 188:6-8
 
a)       If one realizes before completing the beracha of "Boneh Verachamov Yerushalayim" (ie. he said "Boruch Attah" but before saying the name of Hashem), one should go back to say Ritzay or Ya'aleh Viyovoi and continue on from there.
 
b)       If one realizes after beginning the beracha of "Boneh Verachamov Yerushalayim" (ie. he said "Boruch Attah Hashem"), one should say the words "Lamdayni Chukecha" in order to avoid saying the name of Hashem for nothing, and then go back to say Ritzay or Ya'aleh Viyovoi and continue on from there on Shabbos or Yom Tov. However, on Rosh Chodesh or Chol Hamoed [or if one is eating a 3rd meal (or more) on Shabbos or Yom Tov] one does not go back.
 
c)       If one realizes after completing the beracha of "Boneh Verachamov Yerushalayim" (ie. he already said the word "Boneh") but before beginning the 4th beracha (of "Hatov Vehamaitiv") one should add the special beracha (cited below) that replaces Ritzay or Ya'aleh Viyovoi.
  
d)       If one realizes after beginning the 4th beracha and said as much as "Boruch Attah Hashem Elokeinu Melech Haolom" one should continue with the words of the special beracha cited below" beginning with the word following "Ha'olom".
 
e)       If one realizes after beginning the 4th beracha and said as much as "Boruch Attah Hashem Elokeinu Melech Haolom Ho'kel" one must repeat the entire Birchas Hamazon on Shabbos and Yom Tov. However, on Rosh Chodesh or Chol Hamoed [or if one is eating a 3rd meal (or more) on Shabbos or Yom Tov] one does not go back.
 
To Replace a skipped "Ritzay":
"Boruch Attah Hashem Elokeinu Melech Haolom Shenosan Shabossos L'menucha Li'amoi Yisroel B'ahavah L'os U'libris, Boruch Attah Hashem Mekadesh Hashabbos".
 
To Replace a skipped "Ya'aleh V'Yovoi" on Yom Tov:
"Boruch Attah Hashem Elokeinu Melech Haolom Asher NosanYomim Tovim L'Amoi Yisroel L'Sosson U'lisimcha Es Yom Chag Ha_____________ (Succos, or Matzos, or Shavuos) Hazeh, Boruch Attah Hashem Mekadesh Yisroel V'Hazamanim".
 
To Replace a skipped "Ritzay and Ya'aleh V'Yovoi" on a Shabbos/Yom Tov:
"Boruch Attah Hashem Elokeinu Melech Haolom Shenosan Shabossos L'menucha Li'amoi Yisroel B'ahavah L'os U'libris V'Yomim Tovim L'Sosson U'lisimcha Es Yom Chag Ha_____________ (Succos, or Matzos, or Shavuos) Hazeh, Boruch Attah Hashem Mekadesh Hashabbos V'Yisroel V'Hazamanim".
 
To Replace a skipped "Ya'aleh V'Yovoi" on Rosh Chodesh:
"Boruch Attah Hashem Elokeinu Melech Haolom Shenosan Roshay Chodoshim L'Amoi Yisroel L'zikaron".
 
To Replace a skipped "Ya'aleh V'Yovoi" on Chol Hamoed:
"Boruch Attah Hashem Elokeinu Melech Haolom Shenosan Moadim L'Amoi Yisroel L'Sosson U'Lisimcha".
 
To Replace a skipped "Ritzay and Ya'aleh V'Yovoi on a Shabbos/Rosh Chodesh:
"Boruch Attah Hashem Elokeinu Melech Haolom Shenosan Shabossos L'menucha V'Roshay Chodoshim L'Zikaron Li'amoi Yisroel B'ahavah L'os U'libris, Boruch Attah Hashem Mekadesh Hashabbos V'Yisroel V'Roshay Chodoshim".
 
Anytime one should be saying one of these berachos but one does not know the proper wording, one must repeat the entire Birchas Hamazon.
 
670. If one fell asleep during bentching and one is not sure what was said, one must begin again from the beginning.
Shulchan Aruch w/Mishnah Brurah 188:6 see Biur Halacha
 
671. If however, one knows for sure that he at least said to a specific point in the bentching, one should begin from that point.
Shulchan Aruch w/Mishnah Brurah 188:6 see Biur Halacha
 
711. If three people ate and bentched together on Shabbos or Yom Tov and then they realized that they forgot to say Ritzay or Ya'aleh Veyovoi, they should repeat the bentching individually, and not repeat the mezuman because they were already yotze the mezuman.
Shulchan Aruch w/Mishnah Brurah 188:9
 
712. If one began Seudah Shlishis on Shabbos and the meal ends after night, even very late, one must still say Ritzay in bentching. Shulchan Aruch w/Mishnah Brurah 188:10
713. The 4th beracha of bentching is M'dirabbonon - Rabbinically enacted when the Jews killed in Beitar were finally permitted proper burial and their corpses did not decompose in the interim. Shulchan Aruch w/Mishnah Brurah 189:1
 
714. In the house of a mourner (c'v) some have a minhag to change the wording of the beginning of the 4th beracha of bentching as stated in Shulchan Aruch. {One should ask a Rov a shailah to determine which minhag one should follow.}
Shulchan Aruch w/Mishnah Brurah 189:1,2
 
715. When one is bentching on a cup of wine, after completing bentching one must make a new beracha of "Borai Pri Hagofen" and one is not yotze with any earlier beracha made during the meal. Shulchan Aruch w/Mishnah Brurah 190:1
 
716. If after making his beracha the one who is leading birchas hamazon pours from his cup into the cups of others, the others may not drink their wine until the leader has drunk from his first. Shulchan Aruch w/Mishnah Brurah 190:1
 
717. The one who is leading the bentching must drink at least "Melo Lugmav" (one cheekful - for an average person this is 1.55 oz/46 ml, or as much as 2 oz/59 ml for a larger person), but to avoid any question of a beracha achronah one should drink at least a "Revi'is" of the wine (an amount equal to 2 eggs, or appx. 3.8 oz/112 ml). Shulchan Aruch w/Mishnah Brurah 190:3
 
718. If one drank at least a Revi'is one must make a beracha achrona. If one drank at least Melo Lugmav but less than a Revi'is there is a machlokes amongst the Rishonim whether one is obligated in a beracha achrona, and therefore one should not make the beracha achrona unless one drank a Revi'is. Shulchan Aruch w/Mishnah Brurah 190:3
 
719. If the one who is bentching with a cup of wine does not want to drink the wine, he should still make the beracha on the wine and then someone else who is part of the mezuman should drink a revi'is of the wine. Shulchan Aruch w/Mishnah Brurah 190:4, see Biur Halacha
 
720. It is a "Mitzvah Min Hamuvchar" - A Mitzvah done in its best way, if all those present taste the wine. {They need not drink any more than a small taste}.
Shulchan Aruch w/Mishnah Brurah 190:4
 
721. Although one may not be simultaneously doing any other activity (including reading, learning Torah, or even any minor physical activity) during Birchas Hamazon, bi'dieved if one did so, he is nevertheless yotze Birchas Hamazon because he is not worse than a drunk who is also yotze. Shulchan Aruch w/Mishnah Brurah 191:3, see Biur Halacha, also see SA185:4,5 MB4,6
 
722. If there is a minyan bentching together and the leader forgot to add "Elokeinu" in the preface to bentching; as long as the others present have not yet responded he should correct himself and say "Elokeinu". Shulchan Aruch w/Mishnah Brurah 192:2
 
723. If there is a minyan bentching together and the leader forgot to see "Elokeinu" but the others answered with "Elokeinu", the leader should not go back to correct himself. Rather, his next response should have "Elokeinu" added as usual, now that he has been reminded.
Shulchan Aruch w/Mishnah Brurah 192:2, see Sha'arei Tshuvah 5
 
724. If the minyan began bentching and the leader and all present forgot to say "Elokeinu", they are yotze Bi'dieved and need not repeat anything. Shulchan Aruch w/Mishnah Brurah 192:2
 
725. Although for birchas Hamotzie at the start of the meal (as well as Kiddush on Shabbos) one may L'chatchila be yotze the beracha by listening to another make the beracha (who has in mind to include others); for Birchas Hamozon each person should bentch for themselves. 
Shulchan Aruch w/Mishnah Brurah 193:1
 
726. If one ate together with someone who does not know how to say Birchas Hamozon, one should tell the person to listen to and repeat after him word by word, and in that manner fulfill his obligation to bentch. Shulchan Aruch w/Mishnah Brurah 193:1
 
727. If one does not know how to read the Birchas Hamozon, and one is unable to even repeat after someone else who is bentching, one should at least listen to the other person saying the Birchas Hamozon and in that manner be yotze Bi'dieved. Shulchan Aruch w/Mishnah Brurah 193:1
 
728. If two people ate bread together and there is a third in the house, it is a mitzvah to try to have the third eat with them to create a mezuman of three to bentch together.
Shulchan Aruch w/Mishnah Brurah 193:1
 
729. Only men over the age of Bar Mitzvah may be counted towards a mezuman or a minyan. Women and children are not counted towards a mezuman.
Shulchan Aruch w/Mishnah Brurah 199:6
 
730. Women who ate together with three or more men become obligated to listen to and answer to the mezuman of the men but a woman may not lead the bentching.
Shulchan Aruch w/Mishnah Brurah 199:7 See Biur Halacha
 
731. Although according to the Shulchan Aruch and many Rishonim three or more women who ate together may make a mezuman, the accepted minhag is that women do not make a mezuman on their own. Shulchan Aruch w/Mishnah Brurah 199:6,7 See Biur Halacha
 
732. If four people ate together, although if one leaves there would still be three left to make a mezuman, all are obligated to remain together to bentch with a mezuman.
Shulchan Aruch w/Mishnah Brurah 193:1 MB9
 
733. If seven people ate together it is a mitzvah for them to try to find three more people in the house (or building) to join with them to make a minyan and bentch with "Elokeinu".
Shulchan Aruch w/Mishnah Brurah 193:1 MB12
 
734. If ten or more people ate together (e.g. at a Bris) all should L'Chatchila bentch together with a minyan. Shulchan Aruch w/Mishnah Brurah 193:1 MB14
 
735. If one ate with ten but must leave early for a Mitzvah purpose (i.e. to perform another mitzvah that will not be available after waiting for the minyan to betnch) one may bentch with a mezuman of three as long as there is at least a minyan left. Shulchan Aruch w/Mishnah Brurah 193:1 MB16
 
736. If one had in mind when he began the meal that he is not joining in with others for a mezuman, he is then permitted to bentch on his own and not wait for the mezuman.
Shulchan Aruch w/Mishnah Brurah 193:1,2
 
737. If someone needs to leave a group of 3 before making a zimun due to an unforeseen circumstance out of his control, or to prevent a monetary loss one may bentch alone and leave the remaining 2 people to bentch on their own without a zimun. Shulchan Aruch w/Mishnah Brurah 191:5 MB31
 
738. When there is a large gathering, the honor of leading the bentching should be given to someone with a loud voice who will be heard by all present so that they may answer amein to his berachos. Shulchan Aruch w/Mishnah Brurah 193:1 MB17