Particles が and は both define the subject of the sentence from English’s perspective, but they work differently. は is called a “topic marker”, and since it is a grammatical function found only in a handful of languages in the world (Japanese, Korean and a few others), you may find it difficult to understand. As always, the best way to learn is learn through many live examples instead of theorizing.
If you are a native speaker of any European language, は is more natural to use for the subject of a sentence.
- 私はアメリカ人だ -> I am American
- 今日は月曜日だ -> Today is Monday
- 彼女はピアノを弾く -> She plays the piano
- 彼は寝ている -> He is sleeping
が is used instead of は, when there is a focus on the identity of the subject. In the following examples, が cannot be substituted with は, because the focus is on the identity of the subject:
- 誰がそう言ったの？ -> Who said so? Who was the one who said so?
- 彼がそう言った -> He said so. It was none other than he who said so
- なんで私がやらなきゃいけないんだ？ -> Why do I have to do it (instead of somebody else)?
- 彼がどう思おうが私には関係無い -> It doesn’t matter to me what he thinks (particularly he)
- この曲は彼女が書いた -> She (not anybody else) wrote this song
- 彼に会うのはこれが最後だ -> This is the last time I see him (There will be no other occasion)
- 彼が欠場するのはチームにとって大打撃だ -> It is a huge blow for the team that (none other than) he misses the game
- 今朝雪が降った -> Snow fell this morning (not rained, not hailed)
- 頭が痛い -> My head hurts
- 財布が見つからない -> My wallet cannot be located. I cannot find my wallet
- 物は良いが値段が高過ぎる -> It is good stuff but the price is too expensive (the price is the issue)
However は is used when you make a comparison/contrast, even though the focus is still on the identity of the subject. In the following examples, は cannot be substituted with が
- 私は同意しない -> (I don’t know what others think but) I don’t agree
- 私は行かない -> (Others may go but) I won’t go
- 彼女は未だそのことを知らない -> She doesn’t know about it yet (but others may)
- 彼女は背が低いが、彼女の妹は背が高い -> She is short while her little sister is tall
- 鼻水が出るが熱は無い -> I have a runny nose but there is no fever
- 雨は未だ降っているが、風は止んだ -> It is still raining, but the wind has stopped
The use of が and は is not so clear-cut in some cases. In the following examples, either は or が can be used, because the focus could vary or there is no definitive one:
- 彼は/がアメリカ人だとは知らなかった -> I didn’t know he was American
- 雨は/が止んだ -> The rain has stopped