Vi skal sætte ballonerne op, før gæsterne kommer.

Skal is a modal verb. Here it means something like have to / are going to / need to.

So:

• Vi sætter ballonerne op = We put up the balloons / We are putting up the balloons
• Vi skal sætte ballonerne op = We have to put up the balloons / We’re going to put up the balloons

In this sentence, skal adds the idea that this is something that must be done before the guests arrive.

Because sætte op is a fixed verb expression, often called a phrasal verb or verb + particle combination. It means put up / set up.

In Danish main clauses, the particle often comes later in the sentence:

• at sætte ballonerne op
• vi sætter ballonerne op
• vi skal sætte ballonerne op

This is very common in Danish. Compare:

• tage af = take off
• skrive ned = write down
• stille op = line up / set up

So op belongs with sætte, even though the object ballonerne comes in between.

Normally, no. The natural word order is:

• Vi skal sætte ballonerne op

With this type of verb, the object usually comes before the particle op.

So for a learner, the safest pattern is:

• verb + object + particle

Examples:

• tage jakken af
• skrive nummeret ned
• hænge billedet op

Ballonerne is the definite plural form, meaning the balloons.

• en ballon = a balloon
• ballonen = the balloon
• balloner = balloons
• ballonerne = the balloons

So the sentence is talking about specific balloons, not just balloons in general.

In Danish, the present tense is very often used for future meaning when the time is clear from context.

So:

• gæsterne kommer can mean the guests are coming / the guests will come

Because the sentence already includes før and clearly refers to a future event, Danish does not need a special future form here.

This is similar to English in sentences like:

• The guests are coming tomorrow

Because after før, you have a subordinate clause, and subordinate clauses usually keep the normal order:

• subject + verb

So:

• før gæsterne kommer = before the guests come

This is different from Danish main clauses, where the finite verb is often in the second position.

Compare:

• Gæsterne kommer snart. = main clause
• før gæsterne kommer = subordinate clause

Danish normally uses a comma before a subordinate clause, and før gæsterne kommer is a subordinate clause.

So the comma helps show the structure:

• Vi skal sætte ballonerne op, før gæsterne kommer.

In English, some learners expect the comma rules to be the same, but Danish punctuation has its own conventions.

Yes, here før means before.

It can be used with:

• a clause: før gæsterne kommer = before the guests come
• sometimes with other time expressions as well

A useful comparison is:

• før = before
• efter = after

So:

• før gæsterne kommer = before the guests arrive/come
• efter gæsterne kommer would not be natural here; instead you would usually say efter at gæsterne kommer/er kommet depending on meaning

The infinitive is at sætte = to put / to set.

After a modal verb like skal, Danish uses the bare infinitive, without at:

• vi skal sætte
• not vi skal at sætte

This is like English:

• we must put
• not we must to put

Common Danish modal verbs that work this way include:

• skal
• kan
• vil
• må
• bør

Yes, very often. The basic meaning of komme is come, but in many contexts it is naturally translated as arrive.

So:

• gæsterne kommer can mean the guests are coming
• or more naturally in this sentence, the guests arrive

Danish often uses komme where English might prefer arrive.

Possibly, depending on the situation.

• sætte ... op is a general way to say put up
• hænge ... op means hang up

If the balloons are literally being hung, hænge ballonerne op can work. But sætte ballonerne op is broader and very natural for decorations in general.

So sætte ... op is a good general expression to learn for decorations, signs, posters, lights, and similar things.

It can suggest both, and the exact nuance depends on context.

• obligation: we have to put up the balloons
• plan/intention: we’re going to put up the balloons

In this sentence, many native speakers would hear a practical sense of this needs to be done before the guests come. So it is often a mix of plan and necessity.

That is very common with skal in Danish.

A rough learner-friendly guide:

• sætte: the æ sounds a bit like the vowel in English cat, but not exactly. The tt is not strongly released like in English.
• gæsterne: starts with the same æ sound. The ending -erne / -rne in connected speech can sound quite reduced.

A rough approximation:

• sætte ≈ SEH-duh or SEH-də
• gæsterne ≈ GEH-stər-nə with very light final vowels

But Danish pronunciation is often much softer and more reduced than these spellings suggest, so it is best to listen to native audio if possible.