Many English-speakers misuse whom or don't use it at all. Whom is for indirect objects while who is for direct objects. For example, "Julie knows the answer" becomes "Who knows the answer?" Whereas "you am writing to Sally" becomes "To whom are you writing?" Generally Whom follows a preposition, "to, with, for, etc." Also try substituting him or her in place of whom to see if it makes sense as whom replaces him or her. Who, however, replaces he or she.