'Who' is the normal form in most situations. 'Whom' should be used with a preposition - 'To whom did he give the book?', 'With whom did he go?' As you can see it is also normally used in questions. It does seem to be coming less common these days - probably through laziness! Hope this helps Bob

Who is used for the subject, whom is used for the object. David (subject) bought a car from Steve (object). Who bought a car from Steve? David bought a car from whom? / From whom did David buy a car? In most conversations people exclusively use "who." Most people in the U.S. don't actually know the proper use of whom.

Who is the subject for a verb, whom is not. "Who paid for dinner?" You don't say "whom paid for dinner?" "Kevin was with whom?" but you could say 'who' in this context though 'whom' is grammatically correct.