Who

Subject pronoun: who

• Who asked him?
• Who told her?

Object pronoun: whom

• The person whom I thought was responsible was not in fact to blame.
• Ask not for whom the bell tolls.
• To whom it may concern:

whom has fallen into disuse except when following a preposition.

Interrogative pronoun

An unknown (wh-) person

• Who did you see?
• Hmm... who(m) should we ask?

Relative pronoun

allows further modification of the preceding noun or pronoun.

• Marla's the one who's supposed to be doing this, not me!
• She's the person who volunteered...