Hún sest á klett og horfir yfir dalinn þar sem mosinn er enn blautur.

They come from different verbs and describe different things:

• sest = sits down, takes a seat
• situr = is sitting

So Hún sest á klett describes the action of moving into a sitting position. If you said Hún situr á kletti, that would describe her already being seated on the rock.

Also, sest is the 3rd person singular present form of setjast.

Because á changes case depending on meaning:

• accusative for movement/change of position: onto
• dative for location: on

Here she is sitting down onto a rock, so Icelandic uses the accusative:

• á klett

Compare:

• Hún sest á klett = she sits down onto a rock
• Hún situr á kletti = she is sitting on a rock

The dictionary form is klettur meaning rock or crag.

In this sentence you see klett, which is the accusative singular form. This is a common masculine noun pattern:

• nominative: klettur
• accusative: klett
• dative: kletti
• genitive: kletts

So the -ur disappears in the accusative.

Dalinn is the accusative singular definite form of dalur (valley).

There are two things happening here:

1. yfir takes the accusative here
2. the noun is definite: the valley, not just a valley

So:

• dalur = valley
• dal = valley, accusative singular
• dalinn = the valley, accusative singular

In this sentence, horfa yfir means something like:

• look over
• look out over
• gaze across

So horfir yfir dalinn means she is looking out over the valley.

It is a very natural Icelandic way to express looking across a landscape. It does not mean she is literally floating above it; it just describes the direction of her gaze.

Here þar sem means where.

It introduces a clause describing the place:

• dalinn þar sem mosinn er enn blautur
• the valley where the moss is still wet

Literally, þar sem is something like there where, but in normal English you usually just translate it as where.

Mosinn is the definite form of mosi (moss), so it means the moss.

Icelandic usually adds the definite article to the end of the noun:

• mosi = moss
• mosinn = the moss

Here it refers to the moss in that specific place, so the definite form sounds natural.

Because adjectives in Icelandic agree with the noun they describe in:

• gender
• number
• case

Here, mosinn is:

• masculine
• singular
• nominative

So the adjective must match:

• blautur = masculine singular nominative

Compare:

• blautur = masculine
• blaut = neuter
• blaut or blautur/blautir/blautar depending on form and context

In this sentence, mosinn er enn blautur means the moss is still wet.

Here enn means still.

So:

• er enn blautur = is still wet

In other contexts enn can also relate to yet or even more/further, but here still is the right meaning.

Because Icelandic, like English, can leave out the subject when two verbs share the same subject and are joined by og (and).

So:

• Hún sest á klett og horfir yfir dalinn

means:

• She sits down on a rock and looks over the valley

Both verbs are understood to have the same subject, hún.

Yes. Both are 3rd person singular present tense, matching hún:

• hún sest = she sits down
• hún horfir = she looks

This kind of present tense is very common in narration, especially when describing a scene vividly.