Leiðin er löng, en hún ratar alltaf aftur niður af hálendinu.

Because -in is the suffixed definite article.

• leið = a path / a route / a way
• leiðin = the path / the route / the way

Icelandic usually adds the to the end of the noun instead of using a separate word like English does.

It agrees with leiðin, which is feminine singular nominative.

The adjective comes from langur = long, but its feminine nominative singular form is löng.

So:

• langur = masculine
• löng = feminine
• langt = neuter

Since leiðin is feminine, Icelandic uses löng.

Because the adjective is being used predicatively after er.

In Leiðin er löng, the adjective describes the subject after the verb to be, so it stays in the nominative and agrees with the subject:

• leiðin er löng

A form like langa would be used in other grammatical environments, not here.

Because leið is a feminine noun, and Icelandic pronouns follow the noun’s grammatical gender.

So even though English would say it, Icelandic says:

• hún for feminine nouns
• hann for masculine nouns
• það for neuter nouns

Here hún refers back to leiðin. It does not mean the path is literally female; it is just normal grammatical agreement.

Ratar is the 3rd person singular present tense of að rata.

A very useful basic meaning is:

• to find one’s way
• to know the way
• to make one’s way

It often suggests successful navigation rather than simple motion. So this is more than just goes down. It gives the idea that the path finds its way back down or makes its way back down.

Yes, although it can feel slightly poetic or descriptive.

Icelandic often allows roads, paths, rivers, and similar things to be described almost as if they move or guide themselves. Here the sentence gives the path a kind of active role: it finds its way back down.

In natural English, this might be translated less literally depending on style, for example as something like the path eventually winds back down from the highlands.

Here it means back, not again.

Aftur can mean either depending on context:

• gera það aftur = do it again
• fara aftur = go back

Because this sentence is about movement with niður af hálendinu, the meaning is clearly back.

They each add something slightly different.

• niður = down
• af = off / from

So niður af hálendinu means something like:

• down off the highlands
• down from the highlands

This is very natural in Icelandic: a directional adverb such as niður is often combined with a preposition that shows where the movement is away from.

Because af takes the dative case.

The noun is hálendi, a neuter noun, and here it appears in the definite dative singular:

• hálendi = highland / highlands
• hálendinu = the highland(s) in the dative

So:

• af hálendinu = from the highlands / off the highlands

Because in Icelandic hálendi is commonly treated as a singular collective noun.

So even though English often uses a plural-style expression like the Highlands, Icelandic can use singular hálendi to refer to the highland interior as one geographical area.

That is why you get:

• hálendið = the highlands
• af hálendinu = from the highlands

Yes, it is completely normal.

The structure is:

• en = but
• hún = subject
• ratar = verb
• alltaf aftur niður = adverbial elements
• af hálendinu = prepositional phrase

So the clause is built in a very standard way. Icelandic main clauses follow a verb-second pattern, and here that works out naturally because the subject comes first:

• hún ratar ...

If another element came first, the verb would still stay in second position, for example:

• en alltaf ratar hún aftur niður af hálendinu

That version is also grammatical, but the original is the more neutral order here.

En means but and introduces a contrast.

The contrast is between the two ideas:

• Leiðin er löng = the path is long
• en hún ratar alltaf aftur niður af hálendinu = but it always finds its way back down from the highlands

So the sentence is saying: even though the path is long, it still reliably comes back down.