は versus が
は marks the topic: what the sentence is about. が marks the subject: who or what does it, as new information. The same sentence can carry either, and the meaning shifts.
は: as for…
は sets the stage; what follows is a comment about it. The thing before は is usually already known to both of you.
私は学生です。
Reading: わたしはがくせいです。
I am a student. (As for me: student.)
東京は大きいです。
Reading: とうきょうはおおきいです。
Tokyo is big. (Talking about Tokyo.)
が: this one, specifically
が points at the subject as the answer to a question, or as new information. Question words (誰, 何, どれ) always take が, and so does the answer.
誰が来ましたか？ 田中さんが来ました。
Reading: だれがきましたか？ たなかさんがきました。
Who came? Tanaka did.
雨が降っています。
Reading: あめがふっています。
It is raining. (What is happening: rain.)
猫が好きです。
Reading: ねこがすきです。
I like cats. (好き, 嫌い, 分かる, できる and ほしい take が.)
The same sentence, two meanings
私は行きます: as for me, I am going (others may not be). 私が行きます: I am the one going (not someone else). は spreads its focus over what comes after; が puts the spotlight on what comes before.
私は行きます。
Reading: わたしはいきます。
I am going. (As for me.)
私が行きます。
Reading: わたしがいきます。
I'll go. (I, not someone else.)
Inside a clause, が
In a subordinate clause (when, because, the one that…) the subject takes が, not は. は belongs to the main sentence.
彼が来たとき、私は寝ていた。
Reading: かれがきたとき、わたしはねていた。
When he came, I was asleep.
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