Stjernerne kommer frem, når det bliver mørkt.

Because stjernerne means the stars.

• stjerner = stars
• stjernerne = the stars

In Danish, the definite article is often attached to the end of the noun instead of being a separate word like the in English.

Here:

• en stjerne = a star
• stjernen = the star
• stjerner = stars
• stjernerne = the stars

So Stjernerne kommer frem means The stars come out / appear.

kommer frem means something like come out, appear, or become visible.

Literally:

• kommer = come / comes
• frem = forward / out / forth

But together, komme frem is a natural expression meaning that something becomes visible or appears.

So in this sentence, Stjernerne kommer frem is best understood as:

• The stars come out
• The stars appear

It does not mean the stars are physically moving toward you.

Danish often uses the present tense for general truths, habits, and things that regularly happen.

So Stjernerne kommer frem, når det bliver mørkt means a general fact:

• The stars come out when it gets dark

This is very similar to English, where we also often use the present tense in this kind of sentence:

• The stars come out when it gets dark not
• The stars will come out when it will get dark

So the Danish present tense here is completely normal.

når is used for when in the sense of something that happens regularly, generally, or in the future.

In this sentence, it means:

• whenever it gets dark
• when it gets dark, as a usual thing

That is why når fits well.

A learner may compare it with da, which is also sometimes translated as when, but da is usually used about a specific event in the past:

• Da jeg var barn... = When I was a child...

So here:

• når det bliver mørkt = when it gets dark / whenever it gets dark

The det here is an impersonal or dummy subject, similar to English it in sentences like:

• It is raining
• It is dark
• It gets cold

Danish often uses det in the same way for weather, time, light, and general conditions.

So:

• det bliver mørkt = it gets dark

The det does not refer to a specific thing. It is just required by the grammar of the sentence.

Because the adjective is in the -t form.

The basic adjective is:

• mørk = dark

But Danish adjectives often add -t in certain situations, including after det in impersonal expressions like this:

• Det er koldt = It is cold
• Det bliver lyst = It gets light
• Det bliver mørkt = It gets dark

So mørkt is the correct form here.

There is an important difference:

• er mørkt = is dark
• bliver mørkt = gets dark / becomes dark

So bliver emphasizes a change of state.

Compare:

• Det er mørkt. = It is dark.
• Det bliver mørkt. = It is getting dark / It gets dark.

In your sentence, the idea is that the stars appear at the point when it changes from light to dark, so bliver mørkt is the natural choice.

Because når det bliver mørkt is a subordinate clause, and Danish word order changes in subordinate clauses.

In a main clause, Danish usually has verb-second word order:

• Det bliver mørkt.

But after a conjunction like når, the clause is subordinate, so the subject usually comes before the verb:

• når det bliver mørkt

So:

• det = subject
• bliver = verb

That is why når bliver det mørkt would not fit here as a normal subordinate clause.

Yes, absolutely.

Both are correct:

• Stjernerne kommer frem, når det bliver mørkt.
• Når det bliver mørkt, kommer stjernerne frem.

The meaning is basically the same.

The main difference is emphasis:

• Starting with Stjernerne puts the focus first on the stars
• Starting with Når det bliver mørkt puts the time condition first: When it gets dark...

Notice the word order in the second version:

• Når det bliver mørkt, kommer stjernerne frem not
• Når det bliver mørkt, stjernerne kommer frem

That is because when a subordinate clause comes first, the main clause still follows the normal Danish rule with the finite verb before the subject.

Yes. In standard Danish, a subordinate clause is usually separated by a comma.

So:

• Stjernerne kommer frem, når det bliver mørkt.

The part beginning with når is a subordinate clause, so the comma is standard written Danish punctuation.

English punctuation is less strict in comparable cases, but in Danish this comma is very normal.

It can be understood as both, depending on context.

As plain language, it is a completely normal sentence meaning:

• The stars come out when it gets dark

But because it talks about stars and darkness, it can also sound a little poetic or descriptive.

Nothing in the grammar is especially poetic, though. The sentence structure and vocabulary are simple and natural Danish.