Who or whom?
The easiest way to remember which of these to use is to remember that "who" is a replacement for subject pronouns (he, she, they) and "whom" is a replacement for the object pronouns (him, her, them).
Who is taking Katie to the prom? He is taking Katie to the prom.
Whom are you voting for in next week's election? I'm voting for him.
Click here to read more about "who" and "whom" in the context of correct grammar.