Choosing between が vs. は in sentence


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?]

And not:

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.]

Instead of:

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.]

And not:

カリナさんは彼を手伝いに行きます。(x)[Karina will go to help him.]




The particle が doesn't just mark the subject of questions, it marks the
subject of sentences too. It is a subject-marking case particle. Sometimes the
subject can also be the topic, hence the common confusion of が vs は. This
topic has been discussed extensively on this site. I recommend searching
through the threads as I'm sure you will find them helpful.

  * [What exactly is the subject?](https://japanese.stackexchange.com/questions/25667/what-exactly-is-the-subject)
  * [Why が is said to be a subject marker?](https://japanese.stackexchange.com/questions/52577/why-%e3%81%8c-is-said-to-be-a-subject-marker)




The particle が doesn't just mark the subject of questions, it marks the
subject of sentences too. It is a subject-marking case particle. Sometimes the
subject can also be the topic, hence the common confusion of が vs は. This
topic has been discussed extensively on this site. I recommend searching
through the threads as I'm sure you will find them helpful.

  * [What exactly is the subject?](https://japanese.stackexchange.com/questions/25667/what-exactly-is-the-subject)
  * [Why が is said to be a subject marker?](https://japanese.stackexchange.com/questions/52577/why-%e3%81%8c-is-said-to-be-a-subject-marker)

