The pattern がいる is used to express the existence or presence of living things – people, animals, insects, and other animate beings. It translates to "there is" or "there are" in English.
The subject (an animate being) is marked with が, and the location where it exists is marked with に.
The typical word order is [Place]に + [Living thing]が + いる, but the topic can come first when using は:
A key distinction in Japanese is that いる is used for animate beings (people, animals), while ある is used for inanimate objects (books, buildings, ideas). Mixing these up is a common beginner mistake.
いる is a る-verb (一段動詞) and conjugates regularly:
Polite: います
Negative: いない
Past: いた
Past polite: いました
Past negative: いなかった
To ask whether someone or something (animate) exists, simply add か (polite) or use rising intonation (casual).
がいる is also used to express that someone has family members, pets, or other living beings – similar to "to have."