がいる

がいる is used to say that a living being exists in or is present at a location. It is used only with animate beings (people, animals). For inanimate objects, ある is used instead. The subject of the sentence is marked with が, and the location (if mentioned) is marked with に. The verb いる is the intransitive form meaning “to exist/be present,” and it changes to いた for past and いない for negative. Typical contexts include mentioning who or what is at a place, or describing someone's presence as part of a scene. Examples: 猫がいる。 (There is a cat.) 公園に子供がいる。 (There are children in the park.) 部屋に犬がいる。 (There is a dog in the room.) 教室には先生がいる。 (In the classroom, there is a teacher.) 部屋には猫がいない。 (There is no cat in the room.) Note: for non-living things you would say ある instead of がいる.