Who and whom
There are a few tricks to deciding when to use who and when to use whom. Who is used when writing about the subject of a clause, or formulating a question:
Giovanni was the one who cared.
Who is there?
Whom is used when referring to the object of the verb in the clause:
The sentence is about the trustworthiness of them (they), not we.
To reduce formality, sometimes we can simply omit it:
A good way to decide whether to use who or whom is to turn the sentence around and see whether you would use he, him or them in the relative clause. Who goes with the non-‘m’ words – he (or she), they; whom goes with the ‘m’ words – him (or her), them:
Giovanni was the one who cared. [You would say he cared, not him cared, so it is who.]
Giovanni, whom I have met, was attending. [You would say I have met him, so it is whom.]
They are the speakers whom we can trust. [You would say we can trust them, so it is whom.]
Always use whom after a preposition:
But it is also acceptable to omit the prepositional phrase entirely:
Greater acceptance of ending sentences with prepositions has reduced the use of whom. See Ending sentences with a preposition:
He was a leader in whom they could put their faith.
He was a leader they could put their faith in.
To whom should I send the message?
Who should I send the message to?