Learn Hardcore Arabic (Modern Standard): الجيران في هذا المبنى يعملون في المستشفى أو يدرسون في الجامعة. - The neighbors in this building work at the hospital or study at the university.

The plural جيران is a broken plural, which means the inside of the word changes instead of just adding a regular ending. This is very common in Arabic. So you usually need to learn:

جار = neighbor
جيران = neighbors

A native English speaker often expects something like a neat -s plural, but Arabic very often uses these irregular-looking plural patterns.

Why does the sentence begin with الجيران instead of a verb?

Because Arabic allows a topic-first structure very naturally.

So الجيران في هذا المبنى... is like saying:

As for the neighbors in this building, they...

This is completely normal in MSA. It puts focus on the neighbors first.

If you started with the verb in formal Arabic, the structure would change, and the agreement pattern can also change. So for learners, this sentence is a good example of a very common noun-first pattern.

Why do يعملون and يدرسون end in -ون?

The ending -ون marks the verbs as third person masculine plural in the imperfect tense.

So:

يعمل = he works
يعملون = they work

and