The difference between "who" and "whom" primarily relates to their grammatical roles in a sentence.
- Who is used as the subject of a verb. For example: "Who is coming to the party?"
- Whom is used as the object of a verb or preposition. For example: "To whom should I address the letter?"
A good way to decide which to use is to rephrase the sentence to see if you would use "he" or "him." If "he" fits, use "who"; if "him" fits, use "whom."
However, in casual conversation, "who" is often used in places where "whom" would technically be correct.
