Á tjaldstæðinu setjum við upp tjaldið áður en það verður myrkur.

Because Icelandic main clauses usually follow a verb-second pattern. That means the finite verb normally comes in the second position, no matter what comes first.

Here, the place phrase Á tjaldstæðinu is put first, so the verb setjum comes next, and the subject við follows it.

Compare:

• Við setjum upp tjaldið... = We put up the tent...
• Á tjaldstæðinu setjum við upp tjaldið... = At the campsite, we put up the tent...

Both are possible, but the sentence you have gives special prominence to at the campsite.

Here it is best translated as at.

Icelandic á often literally means on, but with many places it corresponds to natural English at. So á tjaldstæðinu is most naturally at the campsite, not on the campsite, even though the Icelandic preposition is á.

This is a good reminder that Icelandic and English prepositions do not match perfectly word for word.

Because á takes different cases depending on whether you mean location or movement.

• á + dative = being in/on/at a place
• á + accusative = movement onto/to a place

Here the meaning is static location: at the campsite, so Icelandic uses the dative.

The noun is tjaldstæði (campsite, neuter), and its dative singular definite form is tjaldstæðinu.

So:

• á tjaldstæðinu = at the campsite
• á tjaldstæðið = onto/to the campsite

In Icelandic, the definite article is usually attached to the end of the noun instead of appearing as a separate word.

So:

• tjaldstæði = campsite
• tjaldstæðinu = the campsite in the dative singular
• tjald = tent
• tjaldið = the tent

So the English word the is built into the noun ending.

The full verb is setja upp, which means to put up, to set up, or to erect.

In Icelandic, verbs like this are often split in a sentence:

• infinitive: að setja upp
• normal main clause: við setjum upp
• with fronting: Á tjaldstæðinu setjum við upp...

The finite verb setjum moves into the main verb position, while the particle upp stays later in the clause.

So setjum við upp tjaldið means we put up the tent.

Setjum is the 1st person plural present tense of setja.

So it means we put or we set.

In context, Icelandic present tense is often used for planned or near-future actions, so here it can also be understood as we will put up or we’re going to put up.

That is very normal in Icelandic.

Tjaldið is the definite singular form of tjald (tent), and here it is the direct object of the verb.

The noun tjald is neuter, and in the singular its nominative and accusative forms are the same. So tjaldið can look the same whether it is a subject or an object.

Here it is the object:

• setjum upp tjaldið = put up the tent

Áður en means before when it is followed by a clause.

So:

• áður en það verður myrkur = before it gets dark

You can think of áður en as a fixed expression here. Even though the two words have their own meanings, together they function as before.

Here það is a dummy subject, like English it in it is raining or it gets dark. It does not refer to a specific thing.

• verður comes from verða, meaning become, get, or sometimes will be
• myrkur means darkness

So það verður myrkur means it gets dark, literally something like it becomes darkness/dark.

This use of það is very common in Icelandic with weather, time, and general conditions.

Because the last part is a subordinate clause, while the first part is a main clause.

In Icelandic:

• main clauses often have verb-second word order
  
  • Á tjaldstæðinu setjum við...
• subordinate clauses usually have more regular subject + verb order
  
  • áður en það verður myrkur

So the difference between setjum við and það verður is a normal grammar difference between main-clause and subordinate-clause word order.