hanamaru
は or が?
は sets the topic — what you are talking about. が points out who or what: the new thing, the answer to a question, what exists, what you like or can do.
は sets the topic (known); が points out who or what (new)
Example Sentences
Are you a student?
Cats love fish
は stands after what the sentence is about — what is already known, what you are talking about: わたしは がくせいです, as for me: student. The rest of the sentence says something about the topic.
が points out who or what. It is new, or it is the answer to a question: ねこが います, there is a cat; だれが いますか — ゆきさんが います, who is there? Yuki. The question word always stands with が, and so does the answer to it.
Some words always have が in front of them: what you like, what you can do, what you are good at. すしが すきです, サッカーが できます, りょうりが じょうずです. Here が is not “who does something” — it points out what is liked, possible or good.
Unsure? Is it something you are already talking about: は. Is it new, an answer, or something that exists: が. Both can stand in one sentence: わたしは すしが すきです — as for me: sushi is liked.