Når februar kommer, er dagene stadig korte, men der kommer langsomt mere lys.

Here når means when.

Danish usually uses når for:

• something that happens regularly
• something expected in the future

So Når februar kommer means when February comes / when February arrives.

It is not da, because da is normally used for a specific event in the past.

It is not hvis, because hvis means if, not when.

In Danish, months are normally not capitalized.

So:

• januar
• februar
• marts

This is different from English, where month names are capitalized.

Month names in Danish usually appear without an article, just like in English in many cases.

So Danish says:

• i februar = in February
• når februar kommer = when February comes

You would not normally say den februar or anything similar here.

Because Når februar kommer is a subordinate clause, and it is followed by the main clause:

• Når februar kommer, = subordinate clause
• er dagene stadig korte = main clause

Danish normally places a comma at that boundary.

This is because Danish main clauses follow the verb-second rule.

The first position in the sentence is taken by the whole subordinate clause:

• Når februar kommer

After that, the finite verb of the main clause must come next:

• er

Then comes the subject:

• dagene

So:

• Når februar kommer, er dagene stadig korte.

This is very common in Danish:

• Når jeg kommer hjem, spiser jeg.
• Hvis det regner, bliver vi inde.

Dagene is the definite plural of dag.

The forms are:

• en dag = a day
• dage = days
• dagene = the days

In this sentence, Danish uses dagene because it means the days in that time of year — the days we are talking about in context.

Because the subject is plural: dagene.

In Danish, predicative adjectives often agree in number:

• Dagen er kort = the day is short
• Dagene er korte = the days are short

So korte is the correct plural form here.

Stadig means still.

So dagene er stadig korte means that the days are short and continue to be short at that point.

It often appears in this kind of position in Danish:

• Han er stadig træt = He is still tired
• Det er stadig koldt = It is still cold

Not exactly, but the basic idea is related.

In:

• Når februar kommer

kommer means arrives / comes around.

In:

• der kommer langsomt mere lys

kommer means something like appears, returns, or starts to be present in greater amount.

Danish uses komme very naturally in both kinds of situations. English often uses different wording, but in Danish this is normal.

This der is a dummy subject, similar to English there in sentences like:

• There is more light
• There comes a time...

In Danish, der is often used when something is being introduced as existing or appearing:

• Der er en bog på bordet = There is a book on the table
• Der kommer flere gæster = More guests are coming

So der kommer langsomt mere lys is a natural way to say that more light is gradually appearing/returning.

Because lys here means light as an uncountable thing.

For uncountable nouns, Danish uses mere:

• mere vand = more water
• mere tid = more time
• mere lys = more light

Flere is used with countable plural nouns:

• flere bøger = more books
• flere dage = more days

A useful extra detail: lys can also mean candle/light as a countable object. In that meaning, flere lys would be possible. But here it clearly means light in general, so mere lys is correct.

Because langsomt is an adverb modifying kommer.

In this sentence, the neutral word order is:

• der kommer langsomt mere lys

That means the increase in light happens gradually.

If you move langsomt, the sentence may still be understandable, but the emphasis can change and it may sound less natural. The version in the sentence is the most standard and natural one.