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.
We sometimes use whom as the object of a verb or preposition, but nowadays we normally use who:
This is George, who you met at our house last year.
This is George’s brother, who I went to school with.
Whom is not used very often in spoken English. Who is usually used as the object pronoun, especially in questions: Who did you invite to the party?
The use of whom as the pronoun after prepositions is very formal:To whom should I address the letter? He asked me with whom I had discussed it. In spoken English it is much more natural to use who and put the preposition at the end of the sentence:Who should I address the letter to? He asked me who I had discussed it with.
In defining relative clauses the object pronoun whom is not often used. You can either use who or that, or leave out the pronoun completely:The family (who/that/whom) I met at the airport were very kind.
The objective case of who pron.: no longer current in natural colloquial speech.
—used as an interrogative or relative —used as object of a verb or a preceding preposition or less frequently as the object of a following preposition (the man whom you wrote to) though now often considered stilted especially as an interrogative and especially in oral use
Whom is only used in written English and in formal spoken English. Who is normally used as the object of a verb or preposition, but immediately after a preposition whom is generally used:
the man with whom she lived.
It would, however, be more natural to say:
the man she lived with