This is explanation from TTMIK. “누구 plus 가” changes to “누가” instead of “누구가” 누구 = who 누구 + 가 = 누구가 --> 누가 Remember that this is ONLY when you are emphasizing ‘who’ as the subject of an action or state. 1.When you want to ask “Who is it?” in Korean, you literally say “it is WHO?” so it becomes: 누구 + 예요 = 누구예요? 2.When you want to ask “(Among these people, none other than) WHO is Jane?” you can say: 누구 + 가 + 제인+ 이에요? = 누가 제인이에요? 3. And when you want to ask “Who did it?”: 누구 + 가 + 했어요? = 누가 했어요? More sample sentences 1. 누가 전화했어요? Who called? 2. 이거 누구예요? Who is this? 3. 어제 누가 왔어요? Who came yesterday? 4. 그거 누가 만들었어요? Who made that? 5. 누가 샀어요? Who bought it?