Important concept #1----use who as the subject of a sentence or clause. For example:
Who let the dogs out? (We still want to know.)
Who washed her hair? (We're guessing it was Sally.)
The woman who showed us to our table was very friendly.
Uh-oh. The first two are straightforward enough, but what's going on with our hostess in example 3? What we have here is a sentence that contains a subordinate clause----who showed us to our table. Observant readers will note that we're actually dealing with two subjects----one for the main sentence (woman) and another for the subordinate clause (who). Even though who is not the main subject of the sentence, it is nonetheless the subject of the clause. Therefore, who is the appropriate form.
Important concept #2----use whom as the direct or indirect object of a sentence or clause. For example:
I will invite whomever I wish.
Whom shall I trust?
And of course, the big one:
Do not ask for whom the bell tolls.
In examples 1 and 2, whomever and whom are direct objects. In example 3, for whom is the indirect object. You grammarians will note that tolls is used as an intransitive verb in this example, which means it does not take a direct object.
We admit that figuring out when to use whom can be tricky. Try rephrasing the sentence or question if you're having trouble deciding which words are the subject and object. For example, if you rephrase "Whom shall I trust" as "I shall trust whom," it makes it easier to see that I is the subject of the sentence (I trust), and that whom must therefore be the object.
Grammar Girl (see the Links page on our website) notes that who is the pronoun that substitutes for the noun he and whom the pronoun that stands in for the noun him. Therefore, she suggests trying both he and him in place of who/whom to see which one sounds correct. For example, would you say "I trust he" or "I trust him?" Obviously, you would say "I trust him." Note that both whom and him end in m, a convenient mnemonic. So if you would use him, whom is the correct choice; conversely, if you would use he, who is the correct pronoun.
Prepositions such as to, for, and in signal indirect objects and are always followed by whom. And watch out for those subordinate clauses. For example:
I will ask whomever.
but
I will ask whoever is interested.
In the first example, whomever is the direct object. But in the second example, whoever is the subject of the subordinate clause whoever is interested.
Using who and whom correctly isn't that difficult if you just think it through. And if you can't bring yourself to do that, by all means give KB COMM a call! We'll be happy to help.