Sada nema mnogo sunca, ali vetar je još jak.

Because nema is the normal Serbian way to say there is no / there isn’t.

It comes from imati (to have), but in expressions of existence it works like this:

• Ima sunca. = There is sun.
• Nema sunca. = There is no sun.

So Sada nema mnogo sunca literally works like Now there isn’t much sun.
Nije would be used with to be, not for this kind of there is/there isn’t idea.

After mnogo (much / a lot of), Serbian normally uses the genitive form of the noun.

So:

• mnogo vode = much water
• mnogo vremena = much time
• mnogo sunca = much sun

That is why you get sunca, not sunce.

Sunca is genitive singular of sunce.

It appears here for two reasons:

1. mnogo usually requires the genitive.
2. nema in this existential sense also commonly goes with the genitive.

So nema mnogo sunca is exactly the kind of structure where genitive is expected.

Sunce is a neuter noun. Its genitive singular form is sunca.

A useful comparison:

• sunce → sunca
• srce → srca

So this is a normal declension pattern for this type of noun.

Serbian usually does not need a separate word like English there in existential sentences.

English says:

• There is...
• There isn’t...

Serbian simply says:

• Ima...
• Nema...

So nema mnogo sunca already means there isn’t much sun without adding any extra word.

Here još means still.

So:

• vetar je još jak = the wind is still strong

In other contexts, još can also mean things like more, another, or yet, so its exact meaning depends on context.

Because Serbian normally uses the verb biti (to be) when an adjective is the predicate.

So:

• Vetar je jak. = The wind is strong.

This is the standard structure.
You cannot normally just say Vetar jak in an ordinary neutral sentence.

Because the adjective has to agree with vetar.

Vetar is:

• masculine
• singular
• nominative

So the adjective must also be:

• masculine
• singular
• nominative

That gives jak.

Compare:

• vetar je jak = the wind is strong
• oluja je jaka = the storm is strong
• duva jako = it blows strongly

Notice that jako is usually an adverb here, not the adjective form matching vetar.

Yes, Serbian word order is somewhat flexible, but vetar je još jak is very natural and neutral.

Here još is placed before jak because it modifies the idea still strong.

You may also hear other orders for emphasis, but this version is the safest standard choice for a learner.

Yes. Sada and sad both mean now.

• Sada is the full form.
• Sad is very common in everyday speech.

So you could also say:

• Sad nema mnogo sunca, ali vetar je još jak.

Both are natural.

Because ali means but and joins two clauses:

• Sada nema mnogo sunca
• vetar je još jak

In standard Serbian, a comma is normally used before ali when it connects clauses like this.

Sometimes yes, but the nuance changes a bit.

• ali = but, stronger contrast
• a = often a softer contrast, or and/while/but depending on context

In this sentence, ali is a very natural choice because the second idea contrasts with the first:

• not much sun, but the wind is still strong

So ali is probably the best default choice here.

Both forms exist in South Slavic standards, but they belong to different pronunciation traditions/standards.

In Serbian, especially Ekavian Serbian, you often get:

• vetar

In Ijekavian varieties, you may see:

• vjetar

So vetar is perfectly normal Serbian. A learner should just know that vjetar may appear in other regional or standard forms nearby.