who / whom
Subject vs. object pronoun.
Who is the subject form, and whom is the object form, used for the object of a verb or a preposition. The pair works exactly like the other personal pronouns, such as I and me, he and him, and they and them: Who signed the letter? Whom did you call? In conversation, nearly everyone uses who in both places, as in Who did you call? In standard written English, or whenever the speaker wants to be formal or technically correct, use whom for the object form.
Examples
UseWho signed the letter? (who is the subject)
UseWhom did you call? (whom is the object of call)
Usethe clerk to whom she wrote (whom is the object of the preposition to)
AvoidWho did you call? (in standard written English or formal speech)