Need help with particle 「が」

Grammar in japanese has always given me the most trouble. I've been studying Kim Tae's Japanese Grammar guide and find it good except I can't understand the particle ga. For the longest time I thought it was the subject marker, and I confused it between the topic, and subject. However, she says this:

"Ok, so we can make a topic using the 「は」 and 「も」 particle. But what if we don't know what the topic is? What if I wanted to ask, "Who is the student?" What I need is some kind of identifier because I don't know who the student is. If I used the topic particle, the question would become, "Is who the student?" and that doesn't make any sense because "who" is not an actual person.

This is where the 「が」 particle comes into play. It is also referred to as the subject particle but I hate that name since "subject" means something completely different in English grammar. Instead, I move to call it the identifier particle because the particle indicates that the speaker wants to identify something unspecified."

Is this true? Is "Ga" really not the subject marker, but rather addressing who a specific/thing person is? And why would one answer with the particle "ga" when answering who that specific person is?

Example:

誰が学生？- Who is the one that is student?
ジムが学生。- Jim is the one who is student.
^^ (Answered with particle "ga". why?)
So, Ga is asking who "that" student is. More specific. If we wanted to use "wa", it would be then:

"Who is a student?"

Not is the "That/the student". Correct?

Thank you

が is indeed the subject marker, and that's why it's "ジムが学生". I think what the author means is that subjects in japanese are often implicit, so you only mark the subject when it presents new information, like in the answer to who is the student. I would suggest you to keep thinking of が as the subject particle, and instead just think of it as that a question word cannot be the topic of a sentence. That's how I think of it, and I think it's fairly correct.