I see this this way: は is a topic particle. You use it, to show the topic. Use it when you are talking about something that the person you are talking to know about, or what was mention earlier. が is a identifier particle. Use it when you wants to identify something unspecified, or when you bring new information to the conversiation. Quote: "The two particles 「は」 and 「が」 may seem very similar only because it is impossible to translate the difference directly into English. For example, 「私は学生」 and 「私が学生」 both translate into, "I am student."* However, they only seem similar because English cannot express information about the context as succinctly as Japanese sometimes can. In the first sentence 「私は学生」, since 「私」 is the topic, the sentence means, "Speaking about me, I am a student". However, in the second sentence, 「私」 is specifying who the 「学生」 is. If we want to know who the student is, the 「が」 particle tells us its 「私」. You can also think about the 「が」 particle as always answering a silent question. For example, if we have 「ジムが魚だ」, we are answering a question such as "Who is the fish?" or "Which person is the fish?" or maybe even "What food does Jim like?" Or given the sentence, 「これが車」, we can be answering the question, "Which is the car?" or "What is the car?" The 「は」 and 「が」 particles are actually quite different if you think of it the right way. The 「が」 particle identifies a specific property of something while the 「は」 particle is used only to bring up a new topic of conversation. This is why, in longer sentences, it is common to separate the topic with commas to remove ambiguity about which part of the sentence the topic applies to."