Himininn er enn skýjaður, en regnboginn sést samt vel.

The ending -inn is the suffixed definite article in Icelandic. In English, you put the before the noun; in Icelandic, the is often attached to the end of the noun.

So:

• himinn = sky
• himininn = the sky

and

• regnbogi = rainbow
• regnboginn = the rainbow

So both nouns are definite here.

Yes. In Himininn er enn skýjaður, himininn is the subject: The sky is still cloudy.

A learner may hesitate because some Icelandic noun forms look the same in more than one case. With himinn, the definite form himininn can look identical in nominative and accusative singular. In this sentence, the verb er and the meaning show that himininn is the subject.

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

Here:

• himininn is masculine singular
• so the adjective is also masculine singular
• therefore: skýjaður

That is why you get:

• himininn er skýjaður = the sky is cloudy

If the noun were feminine or neuter, the adjective would change form.

Yes. Ský means cloud, and skýjaður means something like cloudy / cloud-covered.

So the adjective is built from the noun for cloud rather than from the noun for sky. That is very natural in Icelandic: the sky is cloudy is expressed with an adjective meaning roughly cloud-filled or cloud-covered.

Enn here means still.

It shows that the situation is continuing:

• Himininn er enn skýjaður = The sky is still cloudy

So enn is about time and continuation: something was already true before, and it is true now as well.

This is a very common question, because English uses still in more than one way.

In this sentence, the two words do different jobs:

• enn = still in the sense of continuing
• samt = nevertheless / anyway / even so

So:

• Himininn er enn skýjaður = The sky is still cloudy
• en regnboginn sést samt vel = but the rainbow is visible nevertheless

A very natural English rendering would be:

• The sky is still cloudy, but the rainbow is still clearly visible
• or better, to show the distinction more clearly:
• The sky is still cloudy, but the rainbow is nevertheless clearly visible

Sést is the 3rd person singular present of sjást, which is the middle-voice form related to sjá (to see).

Very roughly:

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

So:

• Regnboginn sést = The rainbow is visible / The rainbow can be seen

The -st ending is a clue that you are looking at a middle-voice form, which often has meanings like be seen, be felt, appear, and so on.

Because with sjást, the thing that is visible is the subject, so it stands in the nominative.

Compare:

• Ég sé regnbogann = I see the rainbow
  Here regnbogann is the object, so it is accusative.

But:

• Regnboginn sést = The rainbow is visible / can be seen
  Here regnboginn is the subject, so it is nominative.

This is a very useful contrast to learn:

• sjá + accusative object
• sjást + nominative subject

Vel literally means well, but with a verb like sést it is often best translated as clearly, easily, or well in the sense of plainly visible.

So:

• sést vel = is easy to see
• is clearly visible
• shows up well

In this sentence, regnboginn sést samt vel means that despite the cloudy sky, the rainbow is still easy to see.

This is normal Icelandic word order.

The basic structure is:

• regnboginn = subject
• sést = verb
• samt = discourse adverb (nevertheless / anyway)
• vel = manner adverb (well / clearly)

So the sentence moves from the main statement to the extra information about how it is seen.

A very literal breakdown is:

• the rainbow
  • is visible
    • nevertheless
      • clearly

English would usually rearrange this a bit more naturally, but the Icelandic order is perfectly normal.