Vi holder os indenfor, hvis det regner i aften.

Because Danish often uses a reflexive pronoun with this expression.

• at holde sig indenfor = to stay inside
• vi holder os indenfor = we stay inside

The os literally means ourselves, but in English we usually do not translate it word-for-word here. It is just part of the natural Danish expression.

Other forms:

• jeg holder mig indenfor = I stay inside
• du holder dig indenfor = you stay inside
• de holder sig indenfor = they stay inside

Here, indenfor means inside or indoors.

So:

• Vi holder os indenfor = We stay inside

A useful distinction:

• indenfor often means inside / within
• udenfor means outside

In everyday Danish, holde sig indenfor is a very common way to say stay indoors.

In this sentence, indenfor is functioning as an adverb, so it is normally written as one word.

Compare:

• Vi bliver indenfor. = We stay inside.
• inden for døren = inside the door / within the door area

So:

• indenfor = adverb, inside
• inden for = preposition + adverb, often followed by something

In your sentence, it is just the adverb, so indenfor is correct.

Hvis means if.

So:

• hvis det regner i aften = if it rains tonight

This is the standard word used for ordinary conditional sentences.

Examples:

• Hvis du kommer, bliver jeg glad. = If you come, I’ll be happy.
• Hvis det sner, bliver vi hjemme. = If it snows, we’ll stay home.

The det here is a dummy subject, like English it in it rains.

Danish needs a subject in the sentence, so weather expressions usually use det:

• det regner = it is raining / it rains
• det sner = it is snowing
• det blæser = it is windy

Just like in English, the det does not refer to a specific thing.

Danish often uses the present tense for future meaning when the time is clear from context.

So:

• det regner i aften literally looks like it rains tonight
• but naturally means it rains tonight / it’s going to rain tonight

Because i aften already tells us the time, Danish does not need a special future form here.

This is very common:

• Vi ses i morgen. = We’ll see each other tomorrow.
• Jeg arbejder på fredag. = I’m working on Friday.

i aften means this evening or tonight, depending on context.

In this sentence, tonight is the most natural translation:

• hvis det regner i aften = if it rains tonight

Related time expressions:

• i dag = today
• i morgen = tomorrow
• i nat = tonight / during the night
• om aftenen = in the evening, generally / in the evenings

Because the clause after hvis follows normal subordinate clause word order.

In Danish subordinate clauses, the subject usually comes before the verb:

• hvis det regner
• fordi han kommer
• når vi er hjemme

So:

• det = subject
• regner = verb

That is the normal pattern after conjunctions like hvis, fordi, når, and at.

Yes, absolutely.

You can say:

• Vi holder os indenfor, hvis det regner i aften.
• Hvis det regner i aften, holder vi os indenfor.

Both mean the same thing.

But notice the word order in the main clause when the hvis-clause comes first:

• Hvis det regner i aften, holder vi os indenfor.

Not vi holder after the comma, but holder vi, because Danish puts the finite verb in second position in main clauses.

Yes, in standard Danish spelling, a comma is normally used before a subordinate clause introduced by hvis.

So:

• Vi holder os indenfor, hvis det regner i aften.

Danish comma rules can be a bit tricky, but with hvis it is very common and expected to write the comma.

Yes. Vi bliver indenfor is also natural and means almost the same thing: We stay inside.

A small difference:

• bliver indenfor focuses on remaining inside
• holder os indenfor is also very common and can sound a little more like keep ourselves indoors

In everyday use, both are fine in many situations.

They are similar, but not always interchangeable.

• inde = inside
• indenfor = inside / indoors
• indendørs = indoors, often a bit more formal or descriptive

Examples:

• Vi er inde. = We are inside.
• Vi bliver indenfor. = We stay inside.
• Det er en indendørs aktivitet. = It is an indoor activity.

In your sentence, indenfor is very natural.

A very literal breakdown would be:

• Vi = we
• holder = keep / stay
• os = ourselves
• indenfor = inside
• hvis = if
• det = it
• regner = rains
• i aften = tonight

So, word-for-word, it is something like:

We keep ourselves inside, if it rains tonight.

But the natural English translation is:

We’ll stay inside if it rains tonight.