في ناموس جنب النهر.

That is a very common question. في can mean in, but in Levantine it can also be an existential word meaning there is / there are.

So here it is not the preposition in. It is introducing the existence of something.

Because it is indefinite here. ناموس means mosquitoes in a general, non-specific sense.

• ناموس = mosquitoes / some mosquitoes
• الناموس = the mosquitoes

Arabic often leaves indefiniteness unmarked in dialect, so you simply see the noun without ال.

In Levantine, ناموس is often used as a collective noun, so it can refer to mosquitoes as a group. If you want to talk about one mosquito, you would usually say ناموسة.

So:

• ناموسة = one mosquito
• ناموس = mosquitoes / mosquito(es) in a general collective sense

جنب means beside, next to, or by. It is a very common location word in spoken Arabic.

It can work on its own, so you do not need an extra word corresponding to English to in next to.

النهر means the river. The definite article ال is used because the sentence is talking about a specific or understood river.

If you wanted next to a river, you could say جنب نهر.
If you mean next to the river, you say جنب النهر.

Roughly:

fii namuus janb / jamb in-nahr

A few notes:

• في is usually pronounced fii
• جنب may sound like janb or jamb in natural speech
• in النهر, the ال blends into the ن, so it sounds like in-nahr or en-nahr

Pronunciation varies a bit by region.

Yes. Many Levantine speakers also use فيه in existential sentences, and in some varieties it may sound even more common in everyday speech.

So both of these can be encountered:

• في ناموس جنب النهر
• فيه ناموس جنب النهر

The exact preference depends on region and speaker.

A very common Levantine way is:

ما في ناموس جنب النهر

This means there aren’t any mosquitoes next to the river or there are no mosquitoes next to the river.

The pattern ما في is extremely common for saying there is no or there are no.