Interrogative pronouns—who, whom, whose, which, and what—introduce questions.
Who Vs. Whom
Who is in subjective case. It can be used in two situations: as the subject of a verb or as the predicate nominative after a linking verb:
Who hosted a Halloween party this year?
It was who?
Whom is in objective case. It can also be used in two situations: as the object of a verb or preposition:
Whom did you invite to your Halloween party?
As whom did you dress for your costume?
If you’re having a hard time figuring out whether the interrogative pronoun is in subjective or objective case, substitute he or him, rewriting the sentence as necessary:
He hosted a Halloween party this year.
You invited him to your Halloween party.
If you would use he in the sentence, then use who. If you would use him in the sentence, use whom. Just remember that the m’s in him and whom go together.
Whose is in the possessive case and thus shows ownership:
Whose costume was best at the party?
Who Vs. Which
When working as interrogative pronouns, who and which can both refer to people, but their uses differ.
Who is general. Use it when anyone could be the answer:
Who wants to go trick-or-treating this year?
Who also asks about the identity of a specific person:
Who is that woman dressed as a witch?
Which is limited, asking for a member of a group:
Which Beatle are you supposed to be?
Which Vs. What
Either which or what can be used when referring to a person or thing:
Which one of you made your own costume?
What kind of candy did you hand out to trick-or-treaters this year?
When used in reference to a person, what asks a question about that person—what they’re like, what they do, etc.:
What do you think of the party’s host?
When used in reference to a thing, what is used broadly to ask for a thing, especially among a set:
What are you supposed to be?
What was the best costume of the night?
Do you have a question about pronouns? Let me know, and I’ll include it in a future installment of Mots Justes’ ongoing series.
The Mots Justes Series on Pronouns
Chicago Manual of Style, The. 15th ed. Chicago: The University of Chicago Press, 2003.
Hacker, Diana, The Bedford Handbook for Writers, 3rd ed. Boston: St. Martin’s Press: 1991.