There are some different functions,in simple sentences, use は because this is the topic of the sentence, eg: Heは is going...tomorrow. In clauses, が is used as the grammatical subject of the clause, eg: 'John knows that Tom is going to..' = 'Johnは Tomがis goingto... knows' For contrast, は is used, eg: 'We don't sent Christmas cards, we send New Year cards' = 'Christmas cardsは don't send, New Year cardsはsend' There are other uses too, but hopefully this will help. :)

Hello, 「が」と「は」の説明は簡単ではありません。 簡単な作文しながら、少しづつ用法を習得するといいかもしれません。 私がお手伝いしますよ。:) You might want to try them when you post notes here on italki. I will help you. :) 「が」（格助詞） This case-marking particle indicates the subject of the verb or adjective verb of the sentence as A⚚P mentioned. ＋ 誰だ！この年賀状を焼いたのは！？ 　ごめんなさい。私がそれを焼きました。 ＋星がきれいだ。 It also indicates the object of the need and the hope. ＋水が飲みたい。 「は」（副助詞） This adverbial particle modifies the verb or the adjective while adding some delicate difference nuance. ＞＞＞'We don't send Christmas cards, we send New Year cards' 私達はクリスマスカードは送らない、（だけど）私達を年賀状は送る。 >>both are は in the case of contrast. 私達はクリスマスカードを送らない。私は年賀状を送る。 >> If you use を, it becomes a normal sentence, not emphasising the contrast... 私は上記のKeithさんの説明を支持します。

Hi, as far as I know, this is a very important feature of Japanese. In particular, "WA" expressions are very used in Japanese and people tend to confuse them with the Subject. - "WA" indicates a Main Reference: it's like saying: --"About this, it's very interesting" - "GA" indicated the Real Subject of the sentence Hope I'm right.