Peysan er hlýrri en ég hélt.

Peysa means sweater.
Peysan means the sweater.

Icelandic usually adds the definite article to the end of the noun instead of using a separate word like English the. So:

• peysa = sweater
• peysan = the sweater

Here the sentence is talking about a specific sweater, so peysan is the right form.

Peysan is nominative singular.

It is nominative because it is the subject of the sentence: the sweater is the thing being described. It is singular because we are talking about one sweater.

The noun peysa is also feminine, which matters for agreement.

Hlýrri is the comparative form of hlýr, which means warm.

So:

• hlýr = warm
• hlýrri = warmer

Icelandic often forms comparatives directly on the adjective, rather than using a separate word like more. So this works much like English warm → warmer.

Yes. Adjectives in Icelandic agree with the noun they describe in gender, number, and case.

Here, peysan is feminine singular nominative, and the comparative adjective appears as hlýrri, which is the correct agreeing form here.

A useful thing to know is that in the comparative, masculine and feminine nominative singular often look the same. So hlýrri fits naturally with peysan.

Here en means than.

It is the normal word used after a comparative:

• hlýrri en... = warmer than...

So Peysan er hlýrri en ég hélt means The sweater is warmer than I thought.

Important: en can also mean but in other sentences, so learners often notice that it has more than one use. In this sentence, because it follows a comparative adjective, it clearly means than.

Hélt is the past tense of halda.

In this kind of sentence, halda means to think, to suppose, or to believe, not physically to hold.

So:

• ég held does not exist here
• ég hélt = I thought / I supposed

This is an irregular verb, so the past tense is not obvious from the dictionary form. That is why it is worth learning as a set:

• halda = to think / suppose
• hélt = thought

Because the sentence combines:

• a present situation: The sweater is warmer
• with a past opinion: than I thought

So:

• er = is — what is true now
• hélt = thought — what you believed earlier

This is completely natural in both Icelandic and English. English also says is warmer than I thought, not necessarily was warmer than I thought.

The rest is simply understood from context.

Just like English can say:

• The sweater is warmer than I thought

without repeating it would be or it was, Icelandic can also stop at ég hélt.

The omitted idea is something like than I thought it was / would be. Icelandic does not need to spell that out here because the comparison already makes it clear.

The word order is very straightforward here:

• Peysan = subject
• er = verb
• hlýrri = predicate adjective
• en ég hélt = comparison clause

So the structure is basically:

The sweater + is + warmer + than I thought

This is one reason the sentence feels fairly accessible to English speakers: the overall order is close to English.

A rough guide is HLEER-ri, but that is only approximate.

A few points:

• ý sounds roughly like English ee, but longer and tenser.
• hl at the beginning is a special Icelandic sound; it is not pronounced exactly like English hl.
• The rr is pronounced with Icelandic r sounds, not like English r.

So an English-friendly approximation is HLEER-ri, but listening to native audio is the best way to get it right.