I understand that if the subject of the question is in question then it will be indicated by が. So we will have:
A: 誰が手伝いに行きますか？ [Who will go to help him a hand?]
A: 誰は手伝いに行きますか？ (x)[Who will go to help him a hand?]
What's odd to me is that in the next sentence that gives answer for A's question, the subject is also indicated by が not は, so:
B: カリナさんが行きます。[Karina will go to help him.]
B: カリナさんは行きます。[Karina will go to help him.]
After search the internet for some time, I found an explanation that says:
When a question word such as "who" and "what" is the subject of a sentence, it is always followed by "ga," never by "wa." To answer the question, it also has to be followed by "ga."
So far so good.
But soon I'm wondering whether this rule is also applicable for affirmative sentences.
So for the following English sentence:
Karina will go to help him.
The Japanese version of it should be:
カリナさんが彼を手伝いに行きます。[Karina will go to help him.]
カリナさんは彼を手伝いに行きます。(x)[Karina will go to help him.]