Garnið er grænt og mjúkt, svo peysan verður hlý og falleg.

In Icelandic, the definite article is usually attached to the noun as a suffix.

• garn = yarn

• garnið = the yarn

• peysa = sweater
• peysan = the sweater

So instead of a separate word like English the, Icelandic often adds the article to the end of the noun.

Because they have to agree with garnið.

Icelandic adjectives change form for:

• gender
• number
• case

Here, garnið is neuter singular nominative, so the adjectives describing it are also neuter singular nominative:

• grænn → grænt
• mjúkur → mjúkt

That -t ending is very common for the neuter singular form of adjectives.

Because they agree with a different noun: peysan.

Peysa is a feminine noun, so the adjectives must be in the feminine singular nominative form:

• hlýr → hlý
• fallegur → falleg

So the sentence shows adjective agreement twice:

• garnið → grænt, mjúkt because garn is neuter
• peysan → hlý, falleg because peysa is feminine

This is a very common question.

The short answer is: because these adjectives are predicative, not attributive.

In this sentence, the adjectives come after er and verður:

• Garnið er grænt
• Peysan verður falleg

After verbs like vera and verða, Icelandic normally uses the strong adjective forms, even when the noun is definite.

Compare:

• græna peysan = the green sweater

  • adjective directly before the noun
  • usually weak form

• Peysan er græn = the sweater is green

  • adjective after er
  • strong form

So grænt, mjúkt, hlý, falleg are expected here.

Verður is the 3rd person singular present tense of verða, which usually means become or will be, depending on context.

So:

• er = is
• verður = becomes / will be

In this sentence, verður shows a result:

• the yarn is green and soft,
• so the sweater becomes / will be warm and beautiful.

If you used er instead, it would sound more like stating a present fact rather than a result of the yarn’s qualities.

Here svo means so, therefore, or as a result.

It connects the two ideas:

• the yarn has certain qualities,
• so the sweater will have certain qualities.

Be aware that svo can mean different things in other contexts, such as:

• then
• so
• very / so much

But in this sentence, it clearly means so / therefore.

They are both in the nominative singular.

That is because both are the subjects of their clauses:

• Garnið er grænt og mjúkt
• peysan verður hlý og falleg

Since the nouns are nominative singular, the predicate adjectives also appear in matching nominative singular forms.

Not especially. The sentence has two straightforward clauses:

• Garnið er grænt og mjúkt
• svo peysan verður hlý og falleg

Each clause uses normal subject-first order:

• subject + verb + adjective(s)

So it feels fairly close to English here.

That said, Icelandic is often described as a verb-second language. In a simple clause where the subject comes first, that looks very familiar:

• Peysan verður hlý

If some other element came first, the verb would still stay in second position. But in your sentence, the order is very natural and uncomplicated for a learner.