Definition of whom pronoun from the Oxford Advanced Learner's Dictionary
whom
pronoun
huːm huːm
(formal)
used instead of ‘who’ as the object of a verb or prepositionWhom 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.
Usage note: whomWhom 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.
huːm 
huːm 
Usage note: whomWhom 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.