は vs が

Use は to mark what the sentence is about. It is often translated as “as for…”

Use が to mark the grammatical subject, especially when identifying who or what did something.

Quick Contrast

私は学生です。

As for me, I am a student.

私が学生です。

I am the one who is a student.

For now, treat は as the marker for the topic already in view, and が as the marker that points directly at the subject.