Ég ætla að sofa betur í nótt.

Ætla að is a very common way to express intention or what someone is going to do.

So Ég ætla að sofa betur í nótt means something like:

• I’m going to sleep better tonight
• I intend to sleep better tonight
• I plan to sleep better tonight

It often suggests a personal decision or expectation, not just a neutral future fact.

After ætla, Icelandic normally uses að + infinitive.

So:

• ætla að sofa = intend to sleep / be going to sleep

Here að works like the English to in to sleep.

You will see this pattern a lot:

• Ég ætla að lesa = I’m going to read
• Hún ætlar að fara = She’s going to leave

Because sofa is in the infinitive form.

The verb that is actually conjugated here is ætla:

• ég ætla = I intend / I’m going to

Then the second verb stays in the infinitive:

• að sofa = to sleep

This is similar to English:

• I want to sleep
• I’m going to sleep

In both languages, only the first verb is finite/conjugated.

Icelandic can talk about the future in several ways, and ætla að is one of the most common.

It does not work exactly like a simple future tense ending. Instead, Icelandic often uses:

• the present tense with future meaning
• munu for a more neutral future
• ætla að for intention or plan

For example:

• Ég sef betur í nótt = I sleep / will sleep better tonight
• Ég mun sofa betur í nótt = I will sleep better tonight
• Ég ætla að sofa betur í nótt = I’m going to / I intend to sleep better tonight

So ætla að is especially natural when the speaker is expressing purpose, plan, or resolve.

Because betur is the comparative adverb of vel (well).

Here it describes how someone will sleep, so you need an adverb:

• sofa vel = sleep well
• sofa betur = sleep better

By contrast, betri / betra / betri are adjective forms meaning better when describing a noun.

Compare:

• Ég sef betur = I sleep better
• Þetta er betra rúm = This is a better bed

So in your sentence, betur is correct because it modifies the verb sofa.

Yes. That is very normal in Icelandic, just like in English.

Ég ætla að sofa betur í nótt naturally implies:

• better than before
• better than last night
• better than I usually do

The comparison is understood from context, so you do not need to state it explicitly.

Í nótt means tonight or more literally in the night / during the night.

It refers to the coming night, or the night around the current time, depending on context.

This is different from:

• í kvöld = tonight / this evening
• á morgun = tomorrow

So:

• í kvöld is the evening
• í nótt is the night, especially the sleeping period

That makes í nótt the natural choice with sofa.

Because the sentence is about sleeping, which normally happens at night, not in the evening.

• í kvöld = this evening / tonight in the evening sense
• í nótt = tonight during the night

So if you say:

• Ég ætla að lesa í kvöld = I’m going to read tonight / this evening
• Ég ætla að sofa í nótt = I’m going to sleep tonight

English uses tonight for both ideas, but Icelandic often distinguishes them more clearly.

That is the normal word order here.

The basic structure is:

• Ég = subject
• ætla = finite verb
• að sofa = infinitive phrase
• betur = adverb modifying sofa
• í nótt = time expression

So:

• Ég ætla að sofa betur í nótt

Putting betur after sofa is very natural because it tells you how the sleeping will happen:

• sleep better

The time phrase often comes later:

• í nótt = tonight

Other word orders are sometimes possible for emphasis, but this version is the most straightforward and natural.

You would usually say:

Ég ætla ekki að sofa betur í nótt.

The important thing to notice is the position of ekki (not):

• it comes after the conjugated verb ætla

So:

• Ég ætla að sofa betur í nótt. = I’m going to sleep better tonight.
• Ég ætla ekki að sofa betur í nótt. = I’m not going to sleep better tonight.

This placement of ekki is a very common Icelandic pattern.

Yes, you could.

• Ég mun sofa betur í nótt = I will sleep better tonight
• Ég ætla að sofa betur í nótt = I’m going to / I intend to sleep better tonight

The difference is mainly one of nuance:

• mun sounds more like a prediction or statement about the future
• ætla að sounds more like intention, decision, or personal plan

So if the speaker is saying this with determination, ætla að is especially natural.