A1
あります and います — existence
How Japanese splits existence into あります for objects and います for living things, and the sentence pattern both share.
English has one verb for existence. Japanese has two, split by animacy: います for things that move under their own power, あります for everything else.
猫がいます。
Neko ga imasu.
There’s a cat.
本があります。
Hon ga arimasu.
There’s a book.
The pattern
The place takes に, the thing takes が, and the verb comes last.
机の上に本があります。
Tsukue no ue ni hon ga arimasu.
There’s a book on the desk.
部屋に誰かいます。
Heya ni dareka imasu.
There’s someone in the room.
Where the line falls
Animals and people take います. Plants take あります — they are alive but they do not move. Vehicles are the interesting case: a bus takes あります parked and often います when you mean the one carrying passengers, because the speaker is thinking of it as something in motion.
犬があります。
犬がいます。
犬 is animate. あります here is not a small error — it sounds like the dog is an object, which is why this one is worth fixing early.
Also have
Both verbs cover possession, which English splits off into a separate verb.
時間がありません。
Jikan ga arimasen.
I don’t have time.
兄弟がいますか。
Kyōdai ga imasu ka.
Do you have siblings?
Siblings are people, so います. The English have tells you nothing about which verb Japanese wants; the thing being had decides.
The plain negative is irregular
ある has ない, not あらない. See negation.