が (ga)
N5ga
The subject particle が — marks who or what performs an action, or introduces new information.
が is the subject particle. It marks the grammatical subject — especially when the information is new or answers who / what.
猫がいます。
Neko ga imasu. — “There is a cat.”
犬がいます。
Inu ga imasu. — “There is a dog.”
Question words pair with が: 誰が (dare ga, who) / 何が (nani ga, what).
For a full comparison with は, see は vs が.