Grammar
N5AI-drafted〜が（主語）
subject marker (new or emphasised information)
Formation
N + が + V／い形容詞／な形容詞
Usage
が marks the subject of a sentence, especially when the subject is new information, is being pointed out, or answers the question "who/what?". It is also used with existence verbs (ある・いる), with natural phenomena, and inside relative clauses. Compare with は, which marks a known topic: 「私は学生です」(as for me) vs 「私がします」(it is I who will do it). Beginners often overuse は; use が when you introduce something for the first time or when the word before it is the answer to a question.
Examples
雨が降っています。
あめがふっています。
It is raining.
あそこに猫がいます。
あそこにねこがいます。
There is a cat over there.
「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?" "Mr. Tanaka came."