These two words must be a couple of the trickiest ones in the English language. It seems as if no speakers, and only some writers, know how to use who and whom correctly. In fact, whom doesn’t even exist in some people’s vocabularies, and it appears to be a word that is quickly disappearing from the English language.
Who is used as the subject of the clause it introduces.
Whom is used as the object of a preposition, as a direct object, or as an indirect object.
A key to remembering which word to use is simply to substitute who or whom with a pronoun. If you can substitute he, she, we, or they in the clause, and it still sounds okay, then you know that who is the correct word to use. If, however, him, her, us, or them sounds more appropriate, then whom is the correct choice for the sentence.
The following are some example sentences that illustrate how to correctly use who and whom.
Example 1 (who): The woman who [not whom] is standing over there is my mother.
Example 2 (whom): Whom are you going out with tonight? (Note that in formal writing, the sentence should be read: “With whom are you going out tonight?”)
Example 3 (whom): The stranded motorist whom I helped was very grateful.
If you substitute she for who in Example 1, it becomes obvious that who is the correct word: “…she [in place of who] is standing over there.” Now take a look at Example 3 above. Take the clause whom I helped and substitute him for whom. If you reverse the order of the words, the clause becomes I helped him. Him is in the place of whom. Remember that although this is a helpful way to distinguish between who and whom, you have to look at just the part of the sentence that begins with who or whom for this trick to work.