What's the difference between "who" and "whom"?
Who is the subject — it does the action. Whom is the object — the action happens to it.
Contexti
The whom form has been quietly fading from spoken English for a century — linguists call it an "object-case pronoun on the way out." It survives in formal writing, in fixed phrases (to whom it may concern), and in cover letters, where using it correctly still reads as careful and literate.
A little moreii
Swap in he or him as a test. If the sentence works with he, use who ("Who called?" — he called). If it works with him, use whom ("To whom did you speak?" — you spoke to him). The m in whom and him is your reminder.
Examplesiii
Whom is calling?
Who is calling?
The caller is doing the calling — subject slot → *who*.
Who should I address this to?
Whom should I address this to?
Addressed *to him* → object slot → *whom*.
Watch foriv
In speech, who is acceptable almost everywhere whom would be strictly correct — nobody says whom did you see. The rule mostly matters in edited prose, legal writing, and formal correspondence.
The full entryv
Subject versus object — the pronoun doing it versus the pronoun it happens to.