は

は (pronounced "wa" when used as a particle, not "ha") marks the topic — what the sentence is about — distinct from the grammatical subject marked by が. Often glossed "as for X," though English has no exact equivalent.

The は/が distinction is the most-asked grammar question in beginner Japanese, and it's a topic-vs-focus contrast rather than a subject-vs-object one. 私は学生です literally says "as for me, [I'm a] student" — the speaker presupposes a discourse about themselves and asserts the predicate. 私が学生です identifies who the student is — answering an implicit "which one of us is the student?".

A reliable rule of thumb: は presents old / established / contrastive information; が introduces new / specific / identificational information. "Wh-" questions (who, what, where) and their answers almost always take が, because the answer is identificational. Existence sentences with ある/いる also take が in the unmarked case.