Of all the tricky grammar topics, who versus whom ranks right up there: get it wrong, and you risk looking like a rube. Get it right and you risk looking pretentious. Get it wrong in a different way and you risk looking like a pretentious rube. So we at Lifehacker, who want to be both right and non-pretentious (but only sometimes succeed), thought we'd do a little research and break down the whole who/whom thing once and for all.
A Quick Lesson on Subjects and Objects
A subject is doing the action of the sentence: She threw the ball.
She is the subject. The object has something done to it: the ball is the object.
Who = Subject, Whom = Object
"Who" functions as a subject: Who wants dinner? Who is going to the ball game?
"Whom" is the object of the verb or the preposition: Whom did you hit? To whom are you speaking?
Use This Mnemonic
A good way to test whether to use who or whom is to swap in he/she or him/her. If he/she works, use who. If him/her works, use whom:
You are following ___?
Because you would say "You are following him," not "You are following he," you know that whom is correct.
___ is taking me to the store?
"She is taking you to the store," not "Her is taking you to the store," so you know that the question should be "Who is taking me to the store?"
My own personal trick for remembering this? Whom and him both end in m. If you can sub in him, it's whom.
Who vs. Whom for Advanced Grammarians
So that's basic who versus whom. Mignon Fogarty, AKA Grammar Girl, offers a brief tutorial on advanced who versus whom that covers adjectival clauses. For example:
I saw the man ___ Joe says followed him around the party.
Who or whom?
The whole second part of the sentence is an adjectival clauses (or a clause that functions as a single adjective): Joe says followed him around the party.
You can remove the "Joe says" because it's a parenthetical, or an aside -- a bit of extraneous information. You're left with "followed him around the party."
And then just apply your standard him/whom trick: Him followed around the party? Nope. He followed him around the party. So use who.
All clear? Or are you all "whom cares?"