When should I use who and when should I use whom?

Who is the subject — the one doing something. Whom is the object — the one something is done to. The distinction is the same one that separates he from him, and that is also the most reliable way to test it.

Rewrite the clause as a statement using he or him. If he fits, use who. If him fits, use whom. The final m in him and whom is a convenient reminder.

Who called? → He called. ✓ To whom did you give it? → You gave it to him. ✓ Who did you invite? → You invited him, so strictly this should be whom did you invite?

That last example shows the real state of the language. In everyday speech and most everyday writing, who has almost entirely displaced whom in object position, and using whom there can sound stiff. It survives most naturally straight after a preposition — to whom, for whom, with whom — which is where it still sounds unremarkable.

The common error is hypercorrection: reaching for whom because it sounds educated, in a position where who is correct. The candidate whom we thought was best is wrong, because the clause is we thought he was best.

A practical position for most writing: use whom directly after a preposition, use who everywhere else, and you will be both correct and unobtrusive.

Formal and legal writing still expects the full distinction.

The same test works inside relative clauses, which is where most real uncertainty lives. The colleague who wrote the report — he wrote it, so who. The colleague whom we consulted — we consulted him, so whom. Several major style guides now advise preferring who in all but the most formal contexts, on the grounds that a correct whom which distracts the reader has cost more than it gained.