Vi slår teltet op, før solen går ned.

Here slår ... op means put up or pitch a tent.

So:

• slå op has several meanings in Danish depending on context
• in this sentence, with teltet (the tent), it means to put the tent up

It does not mean literally hit the tent up. This is a verb + particle expression, similar to English put up.

Because slå op is a particle verb.

In Danish, these are often split in a sentence:

• slår = the finite verb
• op = the particle

With a noun object, the object often comes between them:

• Vi slår teltet op.

Compare the basic expression:

• at slå et telt op = to put up a tent

So the separation is normal Danish word order.

Because Danish usually makes nouns definite by adding the article to the end of the noun.

• et telt = a tent
• teltet = the tent

So -et is the definite ending for a singular neuter noun.

English uses a separate word (the), but Danish often uses a suffix instead.

Because Danish uses the definite form here: solen = the sun.

• en sol = a sun
• solen = the sun

In this sentence, we are talking about the specific sun in the sky, so the definite form is natural.

Also notice that sol is a common-gender noun, so its definite ending is -en, not -et.

Går ned literally means goes down, and here it is the normal Danish way to say that the sun sets.

So:

• solen går ned = the sun goes down / the sun sets

This is very idiomatic and common in Danish.

Because Danish often uses the present tense for future events when the timing is clear from context.

So Vi slår teltet op, før solen går ned can mean:

• We put up the tent before the sun goes down
• We’re putting up the tent before the sun goes down
• We will put up the tent before the sun goes down

The exact English translation depends on context, but the Danish present tense is completely normal here.

Før means before and introduces a subordinate clause.

The sentence has two parts:

• Vi slår teltet op = main clause
• før solen går ned = subordinate clause

So før links the two actions in time: the tent goes up before the sun sets.

Because after før, you have a subordinate clause, and subordinate clauses do not use the normal main-clause verb-second pattern.

So the order is:

• solen = subject
• går = verb
• ned = particle/adverb

That gives:

• før solen går ned

If the før-clause comes first, then the main clause shows inversion:

• Før solen går ned, slår vi teltet op.

Notice slår vi, not vi slår.

The comma marks the start of the subordinate clause.

So:

• Vi slår teltet op, før solen går ned.

This is correct Danish punctuation.

You may also see:

• Vi slår teltet op før solen går ned.

That version is also accepted in modern Danish, depending on the comma system being used. So the comma before før is common and correct, but not always mandatory in every style.

It can mean either, depending on context.

Danish does not usually make the same clear simple present / present progressive distinction that English does.

So:

• Vi slår teltet op can mean We put up the tent
• or We are putting up the tent

In this full sentence, English often prefers:

• We’re putting up the tent before the sun goes down
• or We’ll put up the tent before the sun goes down

Yes, both are irregular and very common.

For slå:

• infinitive: slå
• present: slår
• past: slog
• past participle: slået

For gå:

• infinitive: gå
• present: går
• past: gik
• past participle: gået

So this sentence is also useful because it shows two important everyday irregular verbs.