Þegar himinninn er dökkur og tunglið fullt, göngum við stundum út á svalirnar og tölum rólega.

Because Icelandic is a verb-second (V2) language in main clauses.

The whole Þegar... clause comes first:

Þegar himinninn er dökkur og tunglið fullt

That counts as the first element of the sentence, so the finite verb of the main clause must come next:

göngum við stundum...

If you started with the subject instead, the normal order would be:

Við göngum stundum út á svalirnar...

So the inversion happens because the sentence begins with the Þegar clause.

Because þegar introduces a subordinate clause, and Icelandic does not normally use main-clause V2 word order inside that kind of clause.

So you get:

þegar himinninn er dökkur
not þegar er himinninn dökkur

A useful rule of thumb is:

• Main clause: often V2
• Subordinate clause after words like þegar, að, ef, þótt: usually more regular subject + verb order

Because Icelandic usually puts the definite article on the end of the noun.

• himinn = sky
• -inn = the

So:

himinn + inn = himinninn

Since the noun already ends in -n, adding the article gives the double nn spelling.

This is very common in Icelandic:

• bíll = car
• bíllinn = the car

Because adjectives in Icelandic must agree with the noun they describe in gender, number, and case.

Here:

• himinninn is masculine singular, so the adjective is dökkur
• tunglið is neuter singular, so the adjective is fullt

So the difference is not about meaning; it is about grammatical agreement.

A rough breakdown:

• dökkur = masculine singular nominative
• fullt = neuter singular nominative

The verb er is understood and left out.

The full version would be:

Þegar himinninn er dökkur og tunglið er fullt...

But Icelandic can omit a repeated verb in coordinated structures when the meaning is clear. So og tunglið fullt is a shorter, natural way of saying the same thing.

For a learner, it is helpful to understand it as:

og tunglið [er] fullt

Because svalir is normally a plural noun in Icelandic, even when English uses a singular word.

So:

• svalir = balcony / balconies, depending on context
• svalirnar = the balcony / the balconies

This is one of those nouns you just have to learn with its usual grammatical number. Icelandic treats it as plural in form.

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

Here there is movement out onto the balcony, so Icelandic uses the accusative:

út á svalirnar = out onto the balcony

If you were talking about being already there, you would usually use the dative:

á svölunum = on the balcony

So compare:

• Við göngum út á svalirnar. = We go/walk out onto the balcony.
• Við sitjum á svölunum. = We sit on the balcony.

Yes, á svalirnar could work, but út adds the idea of moving outward, usually from indoors to outdoors.

So:

• ganga á svalirnar = go/walk onto the balcony
• ganga út á svalirnar = go/walk out onto the balcony

In this sentence, út makes the scene more vivid and natural.

They are both first person plural present tense forms:

• við göngum = we walk / we go
• við tölum = we speak / we talk

The endings tell you the subject is we, and Icelandic often changes the stem vowel in these forms because of u-umlaut.

For example:

• tala → tölum
• ganga → göngum

So the -um ending is the important person/number marker, and the vowel change is a normal sound change that goes with it.

Stundum means sometimes. It is an adverb.

Its position is natural because of the V2 structure:

göngum við stundum út...

The main clause starts with the verb göngum, then the subject við, and then the adverb stundum.

Without the fronted Þegar clause, a very normal order would be:

Við göngum stundum út á svalirnar...

So the word itself is simple; its position is mainly affected by the sentence structure.

Because rólega is an adverb, and it describes how the action happens:

tölum rólega = speak quietly / calmly

It comes from the adjective rólegur = calm, quiet.

Very often, Icelandic adverbs are formed with -a:

• rólegur → rólega
• hægur → hægt
• skýr → skýrt

So here rólega modifies the verb tölum, not a noun.

It has a habitual meaning here.

Because of stundum and the general setup, the sentence is not talking about one specific moment. It means something like a repeated or typical action:

whenever these conditions happen, we sometimes go out and talk quietly

So Icelandic present tense, just like English present tense, can describe:

• what is happening now
• what happens regularly
• what is generally true

Here it is the regular/habitual use.