Banyak nyamuk dekat sungai.

Yes. Banyak means many or a lot of, and it commonly comes before the noun:

• banyak nyamuk = many mosquitoes
• banyak orang = many people
• banyak rumah = many houses

So Banyak nyamuk dekat sungai literally starts as Many mosquitoes...

Malay often leaves out words that English requires, especially in simple descriptive sentences.

So:

• Banyak nyamuk dekat sungai. = There are many mosquitoes near the river.

Malay does not always need a direct equivalent of there are.

You can also say:

• Ada banyak nyamuk dekat sungai.

This also means There are many mosquitoes near the river, and the ada makes the existential idea more explicit. Both are understandable, but the version without ada is very natural in everyday speech.

By itself, nyamuk can mean mosquito or mosquitoes, depending on context. Malay nouns usually do not have separate singular and plural forms the way English does.

In this sentence, banyak already tells you it is plural:

• nyamuk = mosquito / mosquitoes
• banyak nyamuk = many mosquitoes

If you wanted to show plurality in another way, Malay can sometimes use reduplication:

• nyamuk-nyamuk = mosquitoes

But here that would be unnecessary, because banyak already makes it clear.

Here, dekat means near, close to, or by.

So:

• dekat sungai = near the river / by the river

In everyday Malay, dekat is very common as a preposition of location.

It can also be used in other ways, for example:

• Rumah saya dekat sekolah. = My house is near the school.
• Mari duduk dekat saya. = Come sit near me.

Because dekat can directly take a noun after it in everyday Malay.

So this is normal:

• dekat sungai = near the river
• dekat rumah = near the house
• dekat sini = near here

You may also hear dekat dengan, but dekat + noun is very common and natural, especially in speech.

Malay does not have articles like English a, an, and the in the same regular way.

So:

• sungai can mean river, a river, or the river
• the exact meaning depends on context

In your sentence, English might translate it as the river if a specific river is understood from context.

If Malay wants to make it more explicitly definite, it can use words like:

• sungai itu = that river / sometimes the river, depending on context

But very often Malay simply leaves it as sungai.

No, not with dekat.

Dekat sungai means near the river or by the river, not in the river.

If you said:

• dalam sungai = in the river

that would mean physically inside the river.

So:

• Banyak nyamuk dekat sungai. = Many mosquitoes near the river.
• Banyak nyamuk dalam sungai. = Many mosquitoes in the river.

The first one is much more sensible.

Not exactly the same.

• dekat sungai = near/by the river
• di sungai = at/in/on the river area, depending on context

If you want to emphasize that the mosquitoes are around the riverbank area, dekat sungai is a very natural choice.

Di sungai can sound broader and sometimes less precise. For mosquitoes, dekat sungai is often the better everyday phrasing if you mean near the river.

Yes. That is a very natural alternative.

Compare:

• Banyak nyamuk dekat sungai.
• Ada banyak nyamuk dekat sungai.

Both mean roughly There are many mosquitoes near the river.

The version with ada feels a bit more explicitly like an existence statement: there are. The version without ada is shorter and very common in conversation.

That is less neutral. The most natural basic order here is:

• Banyak nyamuk dekat sungai.

Starting with banyak makes many mosquitoes a single phrase.

If you say:

• Nyamuk banyak dekat sungai

it may still be understandable, but it sounds more marked or less standard as a basic learner sentence. A native speaker might use unusual word order in certain contexts, but the original sentence is the safer pattern to learn first.

It can be more than one thing.

In this sentence, dekat is a preposition meaning near:

• dekat sungai = near the river

But dekat can also behave like an adjective meaning near/close:

• Rumah itu dekat. = That house is near.

So the same word can appear in slightly different roles depending on the sentence.

A useful approximation is:

• nyamuk ≈ nya-mook

The ny sound is like the ny in Spanish señor, or roughly like the ni sound in onion for English speakers.

So it is not pronounced like plain English n + y separately. It is a single blended sound: nya-muk.

It sounds very natural and conversational.

• Banyak nyamuk dekat sungai. = everyday spoken Malay

If you wanted something more formal, you might choose a more formal word than dekat, such as:

• Banyak nyamuk berhampiran sungai. = There are many mosquitoes near the river.

But in normal speech, the original sentence is completely fine and very common in style.