Just curious, but what's the correct phrase? "For who?" Or "for whom?" What's the main difference?

Native speaker’s answer
Rebecca
Here, the correct phrase would be "for whom," but using "who" instead of "whom" is a very common mistake amongst native English speakers. The difference is that "whom" is used for the object of the sentence, and "who" is the subject of the sentence. Ex: A: Who did you go with? = Whom did you go with? B: I went with her. Ex: A: For whom is this present for? B: The present is for him. Ex: A: Who's going to the party?