Who cares? They're obviously interchangeable, right? Actually, no. The grizzled gurus of grammar have decreed that that and which serve specific grammatical roles. So let's humor them!
Incorrectly substituting which for that is one of the most common grammatical errors we encounter. There's something aristocratically British about it: "Oh, I say! The Bentley which is parked by the Queen's belongs to me!"
Perhaps we should get over ourselves, OK?
The technical truth is that which introduces non-restrictive or parenthetical clauses, and that comes before restrictive clauses. If you know what those are, great; you can move on. But if you're still scratching your head, here are some tips for telling them apart.
Important concept #1----a restrictive clause is essential to the primary meaning or message of the sentence. For example:
I dislike typing on keyboards that aren't mechanical.
In this example, the italicized section is the restrictive clause, correctly introduced by that. Note that if the italicized clause is left out, the entire meaning of the sentence changes----it now indicates that I dislike typing on all keyboards, not just on those that are not mechanical. Big difference, eh?
Always use that to introduce restrictive clauses. Always! Don't even think about using which. Also note that our example contains no commas----do not use commas to set off restrictive clauses.
Important concept #2----a non-restrictive clause is an idea that isn't essential or integral to the primary meaning of the sentence. It always begins with which. For example:
My keyboard, which I keep slightly off-center relative to my monitor, is not ergonomic.
In this example, the italicized section beginning with which is the non-restrictive clause----it has absolutely no bearing on the primary message that my keyboard is not ergonomic. Yes, it contains additional, fascinating information about my computer setup, but we can all agree that positioning one's keyboard to the left (or even the right) of the monitor has nothing to do with the keyboard's inherent ergonomic virtues or lack thereof.
Keen-eyed readers will note that commas set off the clause in our example. These commas are another clue that we're dealing with a non-restrictive clause. They visibly separate the secondary information from the primary message of the sentence. You should always precede which with a comma when constructing a non-restrictive clause. A second comma may or may not be necessary, depending on the sentence.
Affect versus effect
Using these two words correctly is frankly more compelling than for our previous pair, because A) you probably spent some late nights in grade school wondering which to use in that big essay on wheat production in South Dakota (relive the angst!), and B) the words are entirely different parts of speech, so misusing them will make you look much more hopeless.
Fortunately, it isn't that difficult.
First, affect is a verb.* Verbs convey action. Affect and action both start with a. Affect=action=verb. What a great mnemonic!
Because affect is a verb, it has to have a subject and an object (e.g., the weather affected our journey), and it can have several endings (e.g., affected, affecting, affects). Also because it's a verb, you can't precede affect with an article (e.g., the, this, her) or adjective (e.g., big, fast, loud); if you do, you have selected the wrong word, and you should use the other word----effect----instead.
Effect is a noun.* It can have only two forms----singular (i.e., effect) and plural (i.e., effects). Sorry, but we have no great mnemonic for effect. If you come up with one, please share!
Here are some examples of the correct use of each word:
The new data affected the trial's results.
Shocked by the effect, he backed away.
The hardware affects the overall performance of the system.
You may be affected by one or more side effects.
* In the interest of full disclosure, you can use affect as a noun and effect as a verb in very specific situations (e.g., legal documents) that are far beyond the realm of normal human communication. Now forget you ever heard us say this.
