Hún klippir eina grein af runnanum af því að laufin eru að visna.

They are doing two completely different jobs.

• af runnanum = off/from the bush
  • Here af is a preposition meaning off or from.
• af því að = because
  • This is a fixed conjunction introducing a reason clause.

So the sentence is not repeating the same word in the same sense. It just happens that Icelandic uses af in both places.

Because the preposition af normally takes the dative case.

The noun is:

• runi / runni? In this sentence the form is based on runnur = bush, shrub

After af, it becomes dative singular:

• runnanum = the bush in the dative singular

The ending -num includes the definite article, so runnanum means the bush, not just bush.

So:

• af runna = off/from a bush
• af runnanum = off/from the bush

Because grein is the direct object of klippir, so it appears in the accusative singular, and the word einn (one) has to agree with it.

grein is a feminine noun, so:

• nominative: ein grein
• accusative: eina grein

That is why you get eina grein here.

A native English speaker often expects one to stay unchanged, but in Icelandic numbers like einn decline for gender, number, and case.

Because the sentence says one branch, not the branch.

• eina grein = one branch / a branch
• greinina = the branch

Using eina already makes the noun specific in quantity, but it is still grammatically indefinite. Icelandic usually does not combine one with the definite article in a sentence like this.

Klippir is present tense, third person singular, from the verb að klippa (to cut, clip).

So:

• ég klippi = I cut / am cutting
• hún klippir = she cuts / is cutting

In Icelandic, the plain present tense can often cover both:

• a general action: she cuts
• an action happening now: she is cutting

Context tells you which is meant.

This is the common Icelandic pattern:

• vera að + infinitive

It often expresses something that is in progress, similar to English be + -ing.

So:

• eru að visna = are withering / are starting to wither / are in the process of withering

Breakdown:

• eru = are
• að visna = to wither

The verb visna means to wither, fade, or dry up in the sense of plants losing freshness.

Because laufin is plural: it means the leaves.

The noun is:

• lauf = leaf / foliage
  • In actual usage, it very often refers collectively to leaves, and the definite plural laufin means the leaves

Since the subject is plural, the verb is plural too:

• laufin eru = the leaves are

So even though some neuter nouns can look unusual to English speakers, the agreement here is normal:

• plural subject → plural verb

It is nominative plural, because it is the subject of eru að visna.

In the clause:

• laufin eru að visna

the thing doing the action of withering is the leaves, so laufin is the subject.

That is why it is nominative.

Af því að is a very common way to say because.

You should usually learn it as a unit:

• af því að = because

In real Icelandic, you may also encounter:

• því að = also because
• sometimes more conversational shortening in speech

But for a learner, it is best to recognize af því að as a standard conjunction introducing a reason clause.

In your sentence:

• af því að laufin eru að visna
• because the leaves are withering

Because eru að visna is the progressive construction.

Compare:

• laufin visna = the leaves wither / are withering
• laufin eru að visna = the leaves are in the process of withering

The version with vera að + infinitive makes the ongoing nature of the action more explicit. It often feels closer to English are withering.

It has a main clause followed by a reason clause.

Main clause

• Hún klippir eina grein af runnanum
• She cuts one branch off the bush

Reason clause

• af því að laufin eru að visna
• because the leaves are withering

So the pattern is:

• main action + reason

This is a very common sentence structure in Icelandic.

Yes, normally Icelandic uses an explicit subject pronoun here.

• Hún klippir ... = She cuts ...

Unlike some languages that often drop subject pronouns, Icelandic usually keeps them unless the subject is understood in a very specific context, such as commands or certain informal continuations.

So for a normal standalone sentence, Hún is exactly what you would expect.

Here are the key dictionary forms:

• hún = she
• að klippa = to cut, clip
• einn = one
• grein = branch
• runnur = bush, shrub
• lauf = leaf / leaves / foliage
• að visna = to wither, wilt

This is useful because many of the forms in the sentence are inflected:

• klippir comes from að klippa
• eina comes from einn
• runnanum comes from runnur
• laufin comes from lauf
• visna is already the infinitive form after að