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.
An unknown (wh-) person
- Who did you see?
- Hmm... who(m) should we ask?
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 one who volunteered...
- Look! It's the guy who was at the train station.
who(m)ever: n'importe qui, qui qu'il soit, quiconque, ...
- Not to be confused with the World Heath Organisation (WHO / OMS)
- or with The Who singing their famous "Who in the world are you" song... ^^
- or with The Hu, for that matter...