Himlen bliver mørkere, når det begynder at regne.

Himlen is the definite singular form of himmel.

• himmel = sky / a sky
• himlen = the sky

In Danish, the definite article is usually added to the end of the noun instead of standing in front of it. So Danish often says himmel + -en rather than using a separate word for the.

The reason it becomes himlen instead of himmelen is that nouns ending in -el often drop the unstressed e before the ending:

• himmel → himlen
• cykel → cyklen

So this is a normal spelling pattern, not an irregular exception you just have to memorize.

Because bliver shows a change, while er just describes a state.

• Himlen er mørk = the sky is dark
• Himlen bliver mørkere = the sky gets/becomes darker

So in this sentence, the sky is not simply being described as dark. It is changing and becoming darker. That is why bliver is the natural choice.

Mørkere is the comparative form of mørk.

• mørk = dark
• mørkere = darker

A very common way to form the comparative in Danish is to add -ere:

• stor → større
• kold → koldere
• mørk → mørkere

So Himlen bliver mørkere literally has the sense of The sky becomes darker.

Når is used for when in the sense of something that happens generally, repeatedly, or as a normal consequence.

In this sentence, it expresses something like:

• whenever it starts to rain
• when it starts to rain

That makes når the right choice.

Useful comparison:

• når = when / whenever
• da = when, but usually for one specific event in the past
• hvis = if

So:

• Når det begynder at regne, bliver himlen mørkere = when/whenever it starts to rain, the sky gets darker
• Da det begyndte at regne... = when it started to rain... on a specific past occasion

This det is an impersonal or dummy subject, much like English it in it is raining or it starts to rain.

Weather expressions in Danish usually need det:

• Det regner = it is raining
• Det sner = it is snowing
• Det blæser = it is windy / it is blowing

So det begynder at regne literally works like it begins to rain.

The det does not refer to a specific thing. It is just there because Danish grammar requires a subject in this kind of sentence.

Because at marks the infinitive, like English to.

• regne = rain
• at regne = to rain

After begynde, Danish normally uses at + infinitive:

• begynde at regne = begin to rain
• begynde at læse = begin to read
• begynde at arbejde = begin to work

So det begynder at regne is the normal structure.

They are both in the present tense.

• bliver = present tense of blive
• begynder = present tense of begynde

In Danish, the present tense is often used for:

• general truths
• repeated events
• near-future or predictable situations

That is why the sentence can use present tense even though it describes what happens at the moment rain starts.

Also, a very common present-tense pattern in Danish is adding -r:

• blive → bliver
• begynde → begynder
• regne → regner

Yes, you could, but the meaning shifts slightly.

• Når det begynder at regne focuses on the moment the rain starts.
• Når det regner means when/whenever it is raining.

So the original sentence emphasizes the beginning of the change. The alternative version is more general and less focused on that starting point.

If the subordinate clause comes first, the main clause changes word order:

• Himlen bliver mørkere, når det begynder at regne.
• Når det begynder at regne, bliver himlen mørkere.

Notice that after the fronted når clause, Danish puts the finite verb first in the main clause:

• bliver himlen mørkere

not

• himlen bliver mørkere

This is a very important Danish word-order rule. When something other than the subject comes first, the finite verb still stays in second position in the main clause.

Yes. Når introduces a subordinate clause.

The clause is:

når det begynder at regne

Inside that clause, the order is the normal subordinate-clause order:

• det = subject
• begynder = finite verb
• at regne = infinitive phrase

As a learner, it is useful to notice that Danish often keeps subordinate clauses quite straightforward internally, but the main clause may change word order if the subordinate clause is moved to the front.