は vs が — Topic vs Subject Marker

は (wa) — Topic marker

Marks what the sentence is about.

例: 私は学生です。(As for me, I am a student.)

が (ga) — Subject marker

Marks the grammatical subject, often used to introduce new information or with question words.

例: 誰が来ましたか。(Who came?)

Tip: は sets the stage, が points at the actor.