* who (subject)
* whom (object)
* whose (possessive)
Let's look at some examples:
* Who is going to the party? (subject)
* Whom did you call? (object)
* Whose car is that? (possessive)
Note: "Who" and "whom" are sometimes used interchangeably in informal English, but "whom" is the grammatically correct form in formal writing when it acts as the object of a verb or preposition.