Stjörnurnar sjást ekki þegar himinninn er skýjaður.

Here sjást means be seen or be visible.

It is the -st form of sjá (to see), and in sentences about visibility Icelandic very often uses sjást where English would say:

• can be seen
• are visible
• show up

So Stjörnurnar sjást ekki means The stars cannot be seen or The stars aren’t visible.

No. In this sentence, sjást does not mean see themselves.

The -st ending in Icelandic can do several jobs. Historically it comes from a reflexive form, but in modern Icelandic it often works like a middle voice or a passive-like form.

So here:

• sjá = see
• sjást = be seen / be visible

Because the subject is stjörnurnar (the stars), the meaning is clearly about visibility, not reflexive action.

Because the subject is plural.

• stjarnan = the star → sést
• stjörnurnar = the stars → sjást

So the verb agrees with the subject in number:

• Stjarnan sést ekki. = The star cannot be seen.
• Stjörnurnar sjást ekki. = The stars cannot be seen.

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

So:

• stjörnur = stars
• stjörnurnar = the stars

and

• himinn = sky
• himinninn = the sky

This is one of the big differences from English. Instead of a separate word like the, Icelandic normally uses a suffix.

Because many Icelandic nouns change their stem vowel in different forms.

The singular is:

• stjarna = star

But the plural is:

• stjörnur = stars
• stjörnurnar = the stars

This kind of vowel change is very common in Icelandic and is part of the noun’s declension pattern. So this is not random spelling; it is a normal grammatical change that learners usually just have to learn with the word.

Both are in the nominative case, because both are subjects.

• stjörnurnar is the subject of sjást
• himinninn is the subject of er skýjaður

So each clause has its own nominative subject:

• Stjörnurnar sjást ekki
• þegar himinninn er skýjaður

In a normal Icelandic main clause, the finite verb comes early, and ekki usually comes after that finite verb.

So:

• Stjörnurnar sjást ekki

is the normal order.

This is different from English, where not is tied to an auxiliary like do, are, or can. Icelandic does not need that kind of support verb here.

Yes, þegar can mean both when and already.

In this sentence it means when, because it introduces a time clause:

• þegar himinninn er skýjaður = when the sky is cloudy

Context tells you which meaning is meant. Here the clause after þegar makes the meaning when unmistakable.

Because after a subordinating conjunction like þegar, Icelandic normally uses regular subordinate-clause order, where the subject comes before the verb.

So:

• þegar himinninn er skýjaður

is the normal pattern.

A learner often notices that Icelandic main clauses like to put the finite verb very early, but subordinate clauses are less like that and often look more familiar to English speakers.

Because it agrees with himinninn.

Himinninn is:

• masculine
• singular
• nominative

So the adjective also appears in masculine singular nominative:

• skýjaður

This is how predicate adjectives work in Icelandic after vera (to be): they agree with the subject.

For comparison:

• himinninn er skýjaður = masculine singular
• nóttin er skýjuð = feminine singular
• húsin eru skýjuð would not work semantically, but grammatically it shows a neuter plural-type agreement pattern

Also, even though himinninn is definite (the sky), the adjective after er is normally not made definite in the same way. So skýjaður is exactly what you expect here.

Yes.

• ský = cloud
• skýjaður = cloudy / cloud-covered

So skýjaður is built from the noun ský. That is a very common pattern in Icelandic: a noun can be used as the base for an adjective.

Yes. That sentence is grammatical and means almost the same thing:

• Stjörnurnar eru ekki sýnilegar þegar himinninn er skýjaður.
• The stars are not visible when the sky is cloudy.

But Stjörnurnar sjást ekki... is especially natural and idiomatic in Icelandic. Icelandic often prefers a verb like sjást in situations where English might prefer be visible.