The interrogative pronouns (who/which/what) introduce questions. ( What is that? Who will help me? Which do you prefer?) Which is generally used with more specific reference than what . If we're taking a quiz and I ask " Which questions give you the most trouble?", I am referring to specific questions on that quiz. If I ask " What questions give you most trouble"? I could be asking what kind of questions on that quiz (or what kind of question, generically, in general) gives you trouble. The interrogative pronouns also act as Determiners : It doesn't matter which beer you buy. He doesn't know whose car he hit. In this determiner role, they are sometimes called interrogative adjectives .