Sok a nedvesség.

Because Hungarian normally leaves out the copula van in the present tense, third person, when the predicate is a noun, adjective, or quantity word.

Here, sok is the predicate, so Hungarian simply says:

Sok a nedvesség.

Not Sok van a nedvesség.

But in other tenses, the verb appears:

• Sok volt a nedvesség. = There was a lot of moisture.
• Sok lesz a nedvesség. = There will be a lot of moisture.

Also, this zero-copula pattern is different from true there is/there are sentences with van, such as:

• Sok nedvesség van a levegőben. = There is a lot of moisture in the air.

The subject is a nedvesség.

The word sok is the predicate part, describing the amount. So structurally, Hungarian is doing something closer to:

• The moisture is a lot

But in natural English, that usually becomes:

• There is a lot of moisture

So even though the English translation may use there is, the Hungarian sentence is built as a normal subject + predicate sentence.

Because Hungarian often uses the definite article with abstract or mass nouns when referring to the thing in the current situation or context.

So a nedvesség here is not necessarily a very literal the moisture in English. It is more like the moisture present / the moisture level / the moisture in this situation.

Hungarian uses the article more often than English in cases like this, so you should not always try to translate a word-for-word.

Formally, yes: a is the definite article, so it literally corresponds to the.

But in natural translation, English usually would not say The moisture is a lot. Instead, it would say something like:

• There is a lot of moisture
• It’s very damp
• There’s a lot of humidity/moisture

So the article is real in Hungarian grammar, but English often expresses the idea differently.

Because in Hungarian, nouns after numbers and quantity words like sok are normally singular.

So Hungarian says:

• sok könyv = many books
• sok ember = many people
• sok víz = a lot of water

not plural forms after sok.

In this sentence, nedvesség is also a mass noun, like moisture in English, so singular is especially natural.

Because Hungarian word order is strongly connected to emphasis and information structure.

Starting with sok puts the focus on the amount:

• Sok a nedvesség. = The important point is that there is a lot of it.

This is the most natural way to say it in many contexts.

If you say:

• A nedvesség sok.

that sounds much more marked, as if you are contrasting the moisture with something else, or commenting specifically on it. It is not the usual neutral wording.

Both can mean roughly There is a lot of moisture, but the structure is different.

• Sok a nedvesség.
  This is a predicative sentence. It feels a bit like The moisture is a lot in structure, though that is not how you would translate it naturally.

• Sok nedvesség van.
  This is an existential sentence with van, more directly like There is a lot of moisture.

Very often, the difference is small. But a rough way to feel it is:

• Sok a nedvesség = the amount of moisture is high
• Sok nedvesség van = a lot of moisture is present

The van version is especially common when you add a place:

• Sok nedvesség van a falban.
• Sok nedvesség van a levegőben.

Yes, it is natural, but it can sound slightly more descriptive or technical than everyday casual speech, depending on context.

If you are talking about weather or air, Hungarians might also say:

• Párás a levegő. = The air is humid.
• Magas a páratartalom. = The humidity is high.

If you are talking about a wall, room, or building, other common sentences might be:

• Nedves a fal. = The wall is damp.
• Nagy a nedvesség. = There is a lot of moisture / the moisture level is high.

So Sok a nedvesség is perfectly fine, but the most natural wording depends on what kind of moisture you mean.

You can say:

• Nem sok a nedvesség. = There isn’t much moisture.

If you want a stronger opposite idea, you could also say:

• Kevés a nedvesség. = There is little moisture.

Again, there is still no present-tense van here, because this is the same kind of zero-copula structure as in the original sentence.