In my opinion, of all the grammar decisions in all the land, deciding whether to use who or whom is one of the most difficult. In technical terms, the difference between the two is:
Who can be used in place of the subject of a clause or question:
Who told you that was okay?
Brian is the one who told me it was okay.
Whom can be used in the place of the object a clause or question:
With whom are you going to the movies?
Andrew is the guy whom you went with last week.
If you ask me, this is still confusing because the second...