Though some people today tend to ignore the difference in these words, careful writers and speakers retain this distinction of nominative and objective pronoun case.
To choose the correct pronoun of who or whom, you need to recognize whether the pronoun is being used as a subject (the nominative form) or an object (the objective form). Who is nominative (for subjects), and whom is objective (for objects).
- Who (subject) left the party last?
- Whom (object) should the invitation go to?
Trick example:
- The gift must be returned by whoever bought it.
- Here the whole clause whoever bought it is the object of the preposition by.
- But whoever remains nominative because it is the subject of that clause.
Another tip is to try substituting a personal pronoun (he, she, him, her, they, them, and others) in place of who or whom. If he, she, or they fits, use who.
- Who is the junior senator from that state?
She is the junior senator from that state.
If him, her, or them fits, use whom.
- The outcome could depend on whom.
The outcome could depend on them.