Dýnan er mýkri en sú gamla, svo ég sef betur núna.

Dýnan means the mattress.

In Icelandic, the definite article is usually attached to the end of the noun instead of being a separate word like English the. So:

• dýna = mattress
• dýnan = the mattress

Here dýnan is the subject of the sentence.

Mýkri is the comparative form meaning softer.

The basic adjective is mjúkur (soft), with feminine mjúk. In the comparative, the stem changes:

• mjúk = soft
• mýkri = softer

So Dýnan er mýkri means The mattress is softer.

Because the sentence is making a comparison.

• mjúk = soft
• mýkri = softer

The comparison is between the current mattress and the old one, so Icelandic uses the comparative form, just like English does:

• The mattress is soft → Dýnan er mjúk
• The mattress is softer than the old one → Dýnan er mýkri en sú gamla

Here en means than.

So:

• mýkri en sú gamla = softer than the old one

This is worth noticing because en can also mean but in other sentences. The meaning depends on context.

Sú gamla means that old one or, more naturally here, the old one.

It is standing in for the old mattress, so Icelandic does not have to repeat dýnan/dýna again.

You can think of it like this:

• sú = that one / the one
• gamla = old

Together: sú gamla = that old one

Because dýna is a feminine noun, the pronoun is also feminine.

Sú is the feminine singular nominative form of the demonstrative pronoun sá, sú, það (that).

Since dýna is feminine, Icelandic uses the feminine form:

• masculine: sá
• feminine: sú
• neuter: það

So sú gamla matches the gender of dýna.

It is also nominative here because it stands in for the noun phrase being compared.

Because the adjective is in its weak form.

The dictionary form is gamall (old). Its feminine singular form in the normal strong pattern is gömul, but after a word like sú, the adjective becomes weak:

• gömul dýna = an old mattress
• sú gamla = that old one
• gamla dýnan = the old mattress

So gamla is used because sú makes the adjective weak.

Because sef is the correct present-tense form of the verb sofa (to sleep) for I.

• að sofa = to sleep
• ég sef = I sleep

This is an irregular verb, so the form changes quite a bit from the infinitive.

Because betur is an adverb, and it describes the verb sef (sleep).

The sentence is not saying that I am better; it is saying that I sleep better.

So:

• betri = better (adjective)
• betur = better (adverb)

Here you need the adverb, just like in English:

• I sleep better
• ég sef betur

Also, betur is the comparative of vel (well), and it is irregular.

Here svo means so or therefore.

It links the two parts of the sentence:

• the mattress is softer
• so I sleep better now

So svo ég sef betur núna means so I sleep better now.

Because svo here is acting as a conjunction joining two clauses, and the second clause keeps normal word order:

• ég sef = I sleep

So:

• ..., svo ég sef betur núna.

For a learner, the important thing is that this is a normal and natural way to say ..., so I sleep better now.

Núna means now.

It shows a contrast with the past situation: the speaker sleeps better now, after changing the mattress.

Without núna, the sentence would still make sense, but it would lose that clear contrast.

Yes. Icelandic could repeat the noun, but sú gamla is more natural and less repetitive here.

For example, the full repetition would be something like en gamla dýnan or another fuller phrase depending on how you want to word it, but sú gamla works like English the old one, which is very natural in comparisons.

So the sentence sounds smoother with sú gamla.