누가 vs. 누구; 누가 vs. 누구가

Reference: (italki.com) (italki.com)

1. (from member mkei)
* 누가 is a shortened word of 누구가 and it is used when you ask some questions. But we don't say 누구가 in many cases. For examples:

* 누가 미국 사람이에요? (0)

* 누구가 미국 사람이에요? (x)

* 누가 전화했어요? (0)

* 누구가 전화했어요? (x)

* 밖에 누가 왔어요. (0)

* 밖에 누구가 왔어요. (x)

* 누가 내 과자를 먹었지? (0)

* 누구가 내 과자를 먹었지? (x)
2. (from member not like that)

- use 누가 when u want to ask about Subject. Eg: 누가 와요?
- use 누구 when u want to ask about Object. Eg: 누구를 좋아해요?
But there is Exception: use 누구 + 세요 for polite way, even if u ask about Subject ^^
Example: when someone Knock on the door u say " 누구세요?"