Fyrir neðan klettinn er grænn mosi, og dalurinn opnast hægt fyrir framan okkur.

Because klettinn is the definite form: the rock.

A few useful pieces:

• klettur = rock / crag
• kletturinn = the rock in the nominative
• klettinn = the rock in the accusative

Here it appears after fyrir neðan, which takes the accusative in this kind of expression, so you get klettinn, not klettur or kletturinn.

So:

• klettur = a rock
• klettinn = the rock, in the form required by the preposition

Fyrir neðan means below / beneath / underneath.

It is a fixed prepositional expression used for location. In this sentence, it tells you where the moss is located:

• Fyrir neðan klettinn = below the rock

It is very common in Icelandic to build location phrases this way:

• fyrir neðan = below
• fyrir framan = in front of

Even though both begin with fyrir, you should learn the whole phrase as a unit.

This is normal Icelandic word order.

The sentence begins with a location phrase:

• Fyrir neðan klettinn = below the rock

When Icelandic puts something like that first, the finite verb usually comes next. This is the usual verb-second pattern.

So the order is:

• Fyrir neðan klettinn
• er
• grænn mosi

Literally, it looks like:

• Below the rock is green moss

English can do this too, but Icelandic uses this structure much more naturally and often.

Because mosi is a masculine noun.

The adjective has to agree with the noun in:

• gender
• number
• case

Here:

• mosi is masculine singular nominative
• so the adjective is also masculine singular nominative: grænn

That gives:

• grænn mosi = green moss

If the noun had a different gender, the adjective form would change.

Icelandic usually does not use a separate word like English the.

Instead, definiteness is often added to the end of the noun:

• mosi = moss
• mosinn = the moss

In this sentence, grænn mosi is indefinite, so it means green moss, not the green moss.

That is why there is no separate article.

Dalurinn means the valley.

It is made from:

• dalur = valley
• dalurinn = the valley

In Icelandic, the definite article is usually attached to the noun as an ending, not written as a separate word.

So:

• dalur = a valley
• dalurinn = the valley

Here the speaker is referring to a specific valley, so the definite form is used.

Opna means to open something.

• Ég opna dyrnar = I open the door

Opnast is the -st form, which often gives an intransitive, middle, or passive-like meaning. Here it means something like:

• open up
• become open
• reveal itself gradually

So:

• dalurinn opnast = the valley opens up

In landscape descriptions, this is very natural Icelandic. It suggests the valley gradually comes into view or unfolds before you.

The -st ending is very common in Icelandic and can do several related jobs. It often makes a verb less directly transitive and more like:

• something happens by itself
• something becomes a certain way
• something is experienced as unfolding

With opna:

• opna = open something
• opnast = open up / become open

So in this sentence, the valley is not actively opening something else. Rather, it opens up in front of the people observing it.

Yes — and that is exactly why it is useful.

In Icelandic, the neuter singular form of an adjective is often used adverbially.

So:

• hægur = slow
• hægt = slowly

Here it describes how the valley opens up:

• dalurinn opnast hægt = the valley opens up slowly

This is very common in Icelandic:

• adjective form in neuter singular
• used like an adverb

Fyrir framan okkur means in front of us.

The pronoun okkur is the oblique form of við (we/us) used after a preposition.

Relevant forms are:

• við = we
• okkur = us

Since prepositions require an object form, Icelandic uses okkur, not við.

So:

• fyrir framan okkur = in front of us

Yes. In this sentence, both work as fixed prepositional expressions:

• fyrir neðan klettinn = below the rock
• fyrir framan okkur = in front of us

It is best to learn them as complete units rather than trying to interpret fyrir by itself each time. Their full meanings are not just a simple sum of the individual words.

That is a style and meaning choice.

• grænn mosi = green moss
  This introduces something as part of the scene, without making it a uniquely identified item.
• dalurinn = the valley
  This refers to a specific valley that the speaker and listener can identify from the context.

This kind of contrast is very natural in descriptive writing:

• first mention a feature of the landscape
• then refer to a more clearly defined landmark

It sounds descriptive and somewhat literary, but still perfectly understandable and natural Icelandic.

A few things give it that feel:

• fronted location phrase: Fyrir neðan klettinn...
• scenic subject: dalurinn
• verb choice: opnast
• slow unfolding image: hægt fyrir framan okkur

So it is not strange or artificial, but it does have a vivid, scenic tone that is common in narrative and travel-style descriptions.