Who and whom are wh-words. We use them to ask questions and to introduce relative clauses.
Who as a question word
We use who as an interrogative pronoun to begin questions about people:
Who makes the decisions here?
Who did you talk to?
We use who in indirect questions and statements:
The phone rang. She asked me who it was.
Can you tell me who I should talk to.
I can’t remember who told me.
Emphatic questions with whoever and who on earth
We can ask emphatic questions using whoever or who on earth to express shock or surprise. We stress ever and earth:
Whoever does she think she is, speaking to us like that? (stronger than Who does she think she is?)
Who on earth has left all this rubbish here? (stronger than Who has left all this rubbish here?)
Who in relative clauses
We use who as a relative pronoun to introduce a relative clause about people:
The police officer who came was a friend of my father’s.
He shared a flat with Anne Bolton, who he married, and eventually they moved to Australia.
Whom is the object form of who. We use whom to refer to people in formal styles or in writing, when the person is the object of the verb. We don’t use it very often and we use it more commonly in writing than in speaking.
We use whom commonly with prepositions. Some formal styles prefer to use a preposition before whom than to leave the preposition ‘hanging’ at the end of the sentence:
Before a job interview it is a good idea, if you can, to find out some background information about the people for whom you would be working. (preferred in some formal styles to … about the peoplewhomyou would be workingfor)
Over 200 people attended the ceremony, many of whom had known Harry as their teacher.
We use it in relative clauses:
She gave birth in 1970 to a boy whom she named Caleb James.
We use it in indirect questions and statements:
He didn’t ask forwhom I had voted.
He told me where he went and withwhom. (preferred in some formal styles to He told me where he went andwho with.)