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누가=누구

The second semester is over, and I still do not understand, when used 누구 or 누가 (I may have already asked this question, I do not remember.)?I understand that 누구 = 누가 who.
Why in the proposal:누구를 만납니까? And 누가 신무을 봅니까?

It's because "누가" is short for "누구+가".

누구 => noun
가 => subject marker meaning it make a noun into a subject.
"누가" is only used as a subject but "누구" can be a subject or object, even predicate.
1) as a subject
누구 + 가 = 누구가 = 누가
2) as an object
누구 + 을/를 = 누구를
3) as a predicate
누구 + 이다 => 누구입니다 => 누구입니까?(questioning)
누구예요?(informal ending)

\^o^/

I am not a professional teacher of Korean, let me explain though.

1. 누가

'누가' is an abbreviation of '누구가'.
'누가'is used as the subject only in a question sentence.

e.g) 누가 이 음식을 만들었나요?
e.g) 누가 한국어를 가르쳐주나요?
e.g) 누가 운동을 잘하나요?
 

2. 누구

'누구' is used as the personal pronoun of a sentence.
It can be divided into three types depending on the sentence.
 

a. It can be used as the predicate.
    e.g) 저 사람은 누구입니까?
    e.g) 여기에서 가장 잘생긴 사람은 누구입니까?
 

b. It can be used as the subject.
    e.g) 누구는 겨울이 좋다고 하고 누구는 여름이 좋다고 한다.

c.  It can be used as the object.
    e.g) 너는 누구를 만나러 가니?
    e.g) 너는 누구를 좋아하니?