The Bottom Line

The difference between who and whom is exactly the same as the difference between I and me, he and him, she and her, etc. Who, like I, he, and she, is a subject – it is the person performing the action of the verb. Whom, like me, him, and her, is an object – it is the person to/about/for whom the action is being done.

Sometimes it helps to rewrite the sentence and/or replace who/whom with another pronoun so that you can see the relationships more clearly.

This is who warned me > He warned me (not “him” warned me)

Jack is the one who wants to go > He wants to go (not “him” wants to go)

This is the man whom I told you about > I told you about him (not about “he”)

Lisa is the girl with whom I’m driving to Maine > I’m driving to Maine with her (not with “she”)