We have seen that the particle は (wa) indicates the subject of a sentence but the particle が (ga) also indicates it.
The particle は (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: dare ga gakuseidesu ka B: watashi ga gakusi desu
A: Who is the student? B: I am the student.
A: なにが むずかしいですか? B: ぶんぽうが 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: どれが おもしろいですか? B: これが 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.
If it doesn’t start with a interrogative pronoun as the subject of a sentence, you can use the particle は.
せんせいは だれですか? Who is A teacher?
だれが せんせいですか? Who is THE teacher?