Who vs. Whom

The pronoun who is always subjective. Use who wherever you would use the subjective pronouns I, he, she, we, or they. It is correct to say Who wants to go? because we would say I want to go or We want to go.

The pronoun whom is always an object. Use whom wherever you would use the objective pronouns me, him, her, us, or them. It is not correct to say Who did you choose? We would say Whom because you choose me or them.

Handy memory aid: Use this they or them method to decide whether who or whom is correct:

they = who
them = whom

Examples:
Who/Whom wrote the letter?
They wrote the letter. Therefore, who is correct.

Who/Whom should I vote for?
Should I vote for them? Therefore, whom is correct.

We all know who/whom pulled that prank.
This sentence contains two clauses: we all know and who/whom pulled that prank. We are interested in the second clause because it contains the who/whom. They pulled that prank. Therefore, who is correct.

We wondered who/whom the book was about.
This sentence contains two clauses: we wondered and who/whom the book was about. Again, we are interested in the second clause because it contains the who/whom. The book was about them. Therefore, whom is correct.

Note: This rule is compromised by an odd infatuation people have with whom—and not for good reasons. At its worst, the use of whom becomes a form of one-upmanship some employ to appear sophisticated. The following is an example of the pseudo-sophisticated whom.

Incorrect: a woman whom I think is a genius

In this case whom is not the object of I think. Put I think at the end and the mistake becomes obvious: a woman whom is a genius, I think.

Correct: a woman who I think is a genius

Learn to spot and avoid this too-common pitfall.