が marks the subject: the person or thing that does the action or is in the state. It is used when the subject is new or is the important information, for example when a question word is the subject (だれが来ましたか) and in the answer (田中さんが来ました). It is also used when you describe something you see or notice right now (雨が降っています), and when you say that something exists (駅の前に銀行があります).
§1Structure
Noun + が + verb / adjective / noun です. Question word as the subject + が (だれが, 何が, どれが, どの人が), and the answer also takes が. Place に + Noun が + あります／います. Inside a clause that modifies a noun, the subject takes が: 母が作ったケーキ.
§2Examples
「だれが来ましたか。」「田中さんが来ました。」
「だれがきましたか。」「たなかさんがきました。」
'Who came?' 'Tanaka-san came.'
雨が降っています。
あめがふっています。
It's raining.
駅の前に銀行があります。
えきのまえにぎんこうがあります。
There is a bank in front of the station.
バスが来ませんね。
バスがきませんね。
The bus isn't coming, is it?
どれがリンさんのかばんですか。
どれがリンさんのかばんですか。
Which one is Linh-san's bag?
その荷物は重いですね。わたしが持ちます。
そのにもつはおもいですね。わたしがもちます。
That luggage is heavy, isn't it? I'll carry it.
これは母が作ったケーキです。
これはははがつくったケーキです。
This is a cake that my mother made.
あ、電車が来た。
あ、でんしゃがきた。
Oh, the train has come.
§3Common mistakes
- Using は after a question word that is the subject: ×だれは来ましたか. A question word asks for new information, so it takes が: だれが来ましたか. The answer keeps が too: 田中さんが来ました. Answering 田中さんは来ました does not fit this question.
- Using を with あります／います because English says 'have' or 'there is a book': ×机の上に本をあります. The thing that exists is the subject, so it takes が: 机の上に本があります.
- Using は for the subject inside a clause that modifies a noun: ×これはわたしは撮った写真です. Inside such a clause the subject takes が: これはわたしが撮った写真です (This is a photo that I took).
§4How it's different from similar grammar
- は (topic)
- は marks a topic that both people already know, and the new information comes after it: 田中さんは来ました (As for Tanaka-san, he came). が marks the subject itself as the new or important information: 田中さんが来ました (It was Tanaka-san who came). That is why a question word that is the subject takes が (だれが来ましたか), but when the question word comes later, the noun before it takes は (これは何ですか).
- が with 好き / 上手 / わかる (object of a state)
- In 雨が降っています, the noun before が is what does the action: the rain falls. In わたしは猫が好きです, the noun before が is the thing the feeling is about (cats), and the person who has the feeling is marked with は. The particle is the same, but in English the first が-noun becomes the subject and the second one becomes the object: 'I like cats.'
§5How this shows up on the JLPT
N5 Mondai 1 often asks you to choose between は, が and を. Look for three signs of が: a question word right before the blank (だれ（ ）, どれ（ ）), the verbs あります／います, and a short clause before a noun (わたし（ ）買った本). In Mondai 2 (sentence ordering), が must come directly after its noun, so a pair like だれ + が is a good place to start. In listening, the answer to a だれが／何が question repeats が.
§6Quick check (3 questions)
1. Choose the particle that fits the blank: あ、見てください。あそこに猫＿＿います。
あ、みてください。あそこにねこ＿＿います。
2. Which sentence correctly says 'This is a photo that my friend took'?
3. Choose the best answer for B. A：だれがこのケーキを作りましたか。 B：＿＿＿。
A：だれがこのケーキをつくりましたか。 B：＿＿＿。