Pronoun: who 1. The person or persons that; the one that "I'm the one who bought it" 2. What or which person or persons? "who gave you the book?" Pronoun: whom 1. Objective case of "who" ("who" is also often used in the objective) "from whom did you learn that?"

Norman gave the correct answer. In casual usage, we don't really use "whom" and it's one of those things a lot of native speakers will get wrong. So if you're not sure, you can play it safe with "who" If the question would be answered with a "he" use who "Who did this?" "He did it" If the question would be answered with a "him" use whom "Whom did you call?" "I called him" But in casual American English we still just say "who did you call?"