Both いる(Iru) and ある(Aru) are "be verb" in Japanese. In English, there is only one verb “be” and different conjugations for different pronouns. But in Japanese, there are two be verbs which are いる(Iru) and ある(Aru) and they are used in different ways.
There are two Kanji for ある(Aru) which are 有る(Aru) and 在る(Aru). The word 有る(Aru) means possession or absence of something, so it is basically used for non-living things.
Examples
小銭なら有るよ
こぜにならあるよ
Kozeni nara aruyo
I’ve got some change.
終電まだ有るかな
しゅうでんまだあるかな
Shuden mada arukana
I wonder if the train is still running.
The word 在る(Aru) means to exist or to be located. This word is often used for non-living things too but is sometimes used for people as well.
Examples
ここに大きな桜の木が在った
ここにおおきなさくらのきがあった
Kokoni ookina sakura no ki ga atta.
There was a big cherry tree here.
ここに在る校舎は今年で築100年だ
ここにあるこうしゃはことしでちく100ねんだ
Kokoni aru kosha wa kotoshi de chiku 100 nen da.
The school building here is 100 years old this year.
いる(Iru) is written as 居る(Iru) and can be used not only for living things but also for things that are moving. For example, when people say 隣の車線に車がある/となりのしゃせんにくるまがある/Tonari no shasen ni kuruma ga aru/There's a car in the next lane, people often imagine the car that is not moving. But when it’s 隣の車線に車が居る/となりのしゃせんにくるまがいる/Tonari no shasen ni kuruma ga iru/There's a car in the next lane, people imagine the car in motion. As you can see, いる(Iru) is widely used to describe the existence of moving things, not just living things.
Other examples
角の家には大きいな犬がいる
かどのいえにはおおきないぬがいる
Kado no ie ni wa ookina inu ga iru.
There's a big dog in the house at the corner.
工事現場で大きなクレーン車がいると倒れないか心配になる
こうじげんばでおおきなくれーんしゃがいるとたおれないかしんぱいになる
Koji genba de ookina crane sha ga iruto taorenaika shinpai ni naru.
When I see a big crane truck at a construction site, I worry if it will collapse.
In conclusion, いる(Iru) and ある(Aru) are differentiated by whether the subject of the sentence is in motion or not. If it is moving, then use the verb いる(Iru) and if not, use the verb ある(Aru). If you are unsure which Kanji to use, then just write in Hiragana as it would not look too weird. The use of words like いる(Iru) and ある(Aru) is considered to be unique to the Japanese language.
Momoka Yamaguchi