Hi, Dominique! In the simplest way possible, "who" is the subject and "whom" is the object. Use this trick: if you can replace the word with I, he, she, or they, it should be "who." If you can replace the word with me, him, her, or them, it should be "whom." I hope this helps! =)

The above answer is correct. Whom is almost never spoken in modern English however. It would seem strange if the word whom was used in a conversation much here. “To whom it may concern” is about the only phrase it is typically used.