Náttfötin eru hlý, svo ég sef betur.

Because -in is the suffixed definite article in Icelandic.

• náttföt = pajamas / nightclothes
• náttfötin = the pajamas

Unlike English, Icelandic usually attaches the to the end of the noun instead of writing it as a separate word.

It is normally a plural noun. That is similar to English pajamas, which is also often treated as plural in form.

So:

• náttföt = pajamas
• náttfötin = the pajamas

Because it is plural, the sentence uses the plural verb eru.

Because eru is the plural present-tense form of vera (to be).

• er = is for singular
• eru = are for plural

Since náttfötin is plural, you say:

• Náttfötin eru hlý = The pajamas are warm

not Náttfötin er hlý.

In Icelandic, adjectives often agree with the noun they describe in gender, number, and case.

Here, náttfötin is:

• neuter
• plural
• nominative

So the adjective hlýr (warm) appears in the matching form hlý.

This is especially common after vera (to be), where the adjective still agrees with the subject:

• Húsið er stórt = The house is big
• Börnin eru glöð = The children are happy
• Náttfötin eru hlý = The pajamas are warm

Because sofa is the infinitive form, meaning to sleep. After ég (I), you need the conjugated present-tense form:

• að sofa = to sleep
• ég sef = I sleep
• þú sefur = you sleep
• hann/hún/það sefur = he/she/it sleeps

So sef is just the correct finite verb form for I.

Yes, it has a stem change in the present tense, so it can feel irregular to learners.

A few useful forms are:

• að sofa = to sleep
• ég sef = I sleep
• við sofum = we sleep
• ég svaf = I slept (past)

The change from sofa to sef is something you mostly just learn as part of the verb’s pattern.

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

So the sentence structure is:

• Náttfötin eru hlý = The pajamas are warm
• svo ég sef betur = so I sleep better

It links cause and result.

Be aware that svo can also have other meanings in other contexts, such as then or so/very, but here it is the conjunction so.

Because betur is an adverb, while betri is an adjective.

Here, betur modifies the verb sef (sleep), so it means better in the sense of sleep better.

• ég sef betur = I sleep better

But betri would describe a noun:

• betri bók = a better book

A useful comparison:

• góður = good (adjective)
• betri = better (adjective)
• vel = well (adverb)
• betur = better (adverb)

Not literally. It means the pajamas.

If you want to say my pajamas, you would normally say:

• náttfötin mín = my pajamas

That said, in real conversation náttfötin may sometimes refer to a specific known pair of pajamas, depending on context. But grammatically, it is still just the definite form: the pajamas.

Because svo here is a coordinating conjunction, and after it the clause keeps normal main-clause order:

• ég sef betur = I sleep better

So:

• Náttfötin eru hlý, svo ég sef betur

This is different from cases where an adverb or another sentence element is moved to the front and Icelandic then often puts the verb earlier because of V2 word order. Here, svo is just linking the two clauses, so ég stays before sef.

Yes, it is natural here because the sentence contains two coordinated clauses:

• Náttfötin eru hlý
• svo ég sef betur

The comma helps show the pause and the connection between the two full ideas. In careful writing, this is the normal way to punctuate a sentence like this.