Det var kun en kort byge, så bagefter satte vi os udenfor igen.

Kun means only or just. In Det var kun en kort byge, it downplays the event: it was not a big storm, just a brief shower.

Its position is natural because it modifies the phrase en kort byge. English often does something similar: It was only a short shower.

A byge is a short burst of rain or other precipitation, usually a shower. It suggests something brief and passing, not steady rain over a long time.

So regn is the general word for rain, while byge is more like a shower or a sudden spell of rain.

Because byge is a common-gender noun in Danish, so it takes en, not et.

So:

• en byge
• en kort byge

If the noun were neuter, you would use et instead.

Also, kort is the correct adjective form here. In the indefinite singular, the adjective is in its basic form. In definite or plural forms, you would get korte, for example:

• den korte byge
• korte byger

Here så means so, therefore, or as a result. It links the two clauses:

• Det var kun en kort byge
• så bagefter satte vi os udenfor igen

So the second part is the consequence of the first: because it was only a short shower, they sat outside again afterwards.

Bagefter means afterwards or after that. It refers to what happened after the shower had passed.

It is a very common word in everyday Danish for sequencing events:

• Først spiste vi, og bagefter gik vi hjem. = First we ate, and afterwards we went home.

This is because Danish main clauses normally follow the verb-second rule. The finite verb comes in the second position of the clause.

In the second clause, bagefter has been placed first for emphasis. That means the verb satte must come next, and the subject vi comes after the verb:

• bagefter = first element
• satte = finite verb in second position
• vi = subject

So:

• så bagefter satte vi os udenfor igen

not:

• så bagefter vi satte os ...

This is very typical Danish word order.

Because sætte sig and sidde are different.

• sætte sig = to sit down, meaning the action of moving into a seated position
• sidde = to be sitting, meaning the state of already being seated

Here the sentence describes an action that happened afterwards: they went and sat down outside again. So satte vi os is the right choice.

Compare:

• Vi satte os udenfor. = We sat down outside.
• Vi sad udenfor. = We were sitting outside.

Because the verb is the reflexive expression sætte sig, which means to sit down.

With different subjects, the reflexive pronoun changes:

• jeg satte mig
• du satte dig
• han/hun satte sig
• vi satte os
• I satte jer
• de satte sig

So os matches vi. It is not especially emphatic here; it is just part of the normal Danish verb pattern.

Udenfor means outside. In this sentence it works as an adverb of place: it tells you where they sat.

• satte vi os udenfor igen = we sat down outside again

This is the normal form to use here. It is a very common everyday word.

Igen means again. It shows that they were outside before, then the shower interrupted that, and afterwards they returned to the same situation.

So udenfor igen means outside again.

Its position at the end sounds very natural in Danish. If you move igen, the sentence can still be possible in some contexts, but the emphasis changes. Here the final position is the most straightforward choice.