Partícula が

We have seen that the particle (wa) indicates the subject of a sentence but the particle (ga) also indicates it.

The particle (wa) cannot be marked interrogative pronouns such as だれ(dare) who, なに(nani) what, どれ(dore) which one, etc.

You have to need the particle が and don’t forget repeat it to indicate the subject when answering.

A: だれ がくせいですか?  B: わたし がくせいです。
A: dare ga gakuseidesu ka   B: watashi ga gakusi desu
A: Who is the student?    B: I am the student.

A: なに むずかしいですか?   B: ぶんぽう むずかしいです。
A: Nani ga muzukashii desu ka   B: bunpou ga muzukashii desu
A: What is difficult?    B: The grammar is difficult.

A: どれ おもしろいですか?  B: これ おもしろいです。
A: dore ga omoshiroi desu ka   B: kore ga omoshiroi desu
A: ¿Which one is more interesting?  B: This is more interesting.
-Note-
If it doesn’t start with a interrogative pronoun as the subject of a sentence, you can use the particle は.
せんせい だれですか? Who is A teacher?
だれ せんせいですか? Who is THE teacher?