who vs. whom
Someone will tell you "whom" is dead. In speech they are very nearly right — nobody says "whom did you call" outside a costume drama. In writing it is alive, and using it wrongly is worse than not using it at all. A misplaced "whom" reads as someone reaching for a formality they have not quite got hold of, which is a more conspicuous error than simply writing "who" throughout.
who
The subject form. Use it when the word is doing the action.
- Who signed the agreement?
- The partner who reviewed it had concerns.
- Who wants to take this one?
- I need to know who approved the change and when.
whom
The object form. Use it when the word is receiving the action, or after a preposition.
- To whom should I address the invoice?
- The client whom we met last week has questions.
- Whom did the committee select?
- For whom is this being prepared? (Formal, and the preposition makes "whom" mandatory rather than optional.)
Why who and whom get mixed up
Who and whom are the same word doing two jobs. Who is the one doing something, the subject. Whom is the one something is done to, or the word after a preposition, the object. It is the same split as he and him.
That gives the most reliable test. Rephrase the question as an answer, using he or him. If the answer uses he, write who. If it uses him, write whom. "Who called?" gives "he called", so who. "To whom should I send it?" gives "send it to him", so whom.
In conversation, many people use who everywhere and nobody minds. In formal writing, and especially straight after a preposition (to whom, for whom, with whom), whom is still expected.
Common mistakes
Whom is responsible for the budget?
✓Who is responsible for the budget?
The answer is "he is responsible", so the subject form: who.
To who should I address the letter?
✓To whom should I address the letter?
Right after a preposition, use the object form: to whom.
Whom do you think will win?
✓Who do you think will win?
"Do you think" is an aside. The real question is "who will win", and the answer is "he will win", so who.
Quick way to remember
Answer the question with "he" or "him." If "he" fits, use "who" — both end in a vowel sound. If "him" fits, use "whom" — both end in "m." "Who called?" — "He called." "You called whom?" — "You called him." It works every time, including in the middle of a sentence.
Questions
Is whom disappearing?
In speech it is much less common than it was, and "who did you meet?" is widely accepted. In formal writing, and after a preposition, whom is still standard.
Does the same rule apply to whoever and whomever?
Yes, but the test applies inside the clause, not to the verb before it: "give it to whoever asks" is right, because whoever is the one doing the asking.