When you are using the word who in a sentence make sure that it requires a subject pronoun (equivalent to he or she ) . In formal English , use whom when a sentence requires an object pronoun (equivalent to him or he) .Contemporary usage , however increasingly favors the use of who in both cases . Strictly speaking , it is correct to use who as the subject of a verb and whom as the object . Also use whom after a preposition . Whom is a word invented to make to make everyone sound like a butler . The use of who or whom in a subordinate clause depends on how the pronoun functions in the clause . The word who is in the nominative case and the word whom is in the objective case .
Details found from the Holt Book.