は vs. が: what's the difference?

は marks the topic; the key information comes after it. が highlights the subject; the key information comes before it.

• わたしは がくせいです。 I am a student (talking about myself).
• だれが きましたか。 — たなかさんが きました。 Who came? — Tanaka came.

When だれ, なに or どこ is the subject of a question, always use が.