whom

Từ điển Oxford Advanced Learner 8th

whom [whom] BrE [huːm] NAmE [huːm] pronoun (formal)
used instead of ‘who’ as the object of a verb or preposition
• Whom did they invite?
• To whom should I write?
•The author whom you criticized in your review has written a reply.
•Her mother, in whom she confided, said she would support her unconditionally.

Grammar Point:
whom
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.
In non-defining relative clauses who or, more formally, whom (but not that) is used and the pronoun cannot be left out: ▪ Our doctor, who/whom we all liked very much, retired last week. This pattern is not used very much in spoken English.