Ég sofna fljótt þegar ég hlusta á tónlist.

Sofna means to fall asleep, while sofa means to sleep.

So:

• Ég sofna fljótt = I fall asleep quickly
• Ég sef vel = I sleep well

This is an important distinction in Icelandic. English sometimes uses sleep in a broad way, but Icelandic clearly separates the action of falling asleep from the state of being asleep.

Yes — and that often confuses learners.

For many Icelandic verbs, the infinitive and the 1st person singular present tense look the same.

Here:

• að sofna = to fall asleep
• ég sofna = I fall asleep

and

• að hlusta = to listen
• ég hlusta = I listen

So even though the form looks like the dictionary form, in this sentence it is finite because of the subject ég and the sentence structure.

Because fljótt is an adverb, and it modifies the verb sofna.

• fljótur / fljót / fljótt are adjective forms meaning quick
• fljótt is also the form used adverbially, meaning quickly

Here it describes how the person falls asleep:

• Ég sofna fljótt = I fall asleep quickly

If you were describing a noun instead, you would use the adjective form that matches the noun’s gender, number, and case.

Here þegar means when and introduces a subordinate clause:

• þegar ég hlusta á tónlist = when I listen to music

A useful thing to know is that þegar can also mean already in other contexts, so its meaning depends on how it is used.

In this sentence, it is clearly the conjunction when.

Because the clause after þegar is a full clause with its own subject and verb.

So Icelandic says:

• Ég sofna fljótt þegar ég hlusta á tónlist

Just like English says:

• I fall asleep quickly when I listen to music

You cannot leave out ég here, because þegar ég hlusta á tónlist needs its own subject.

Because the verb hlusta normally takes the preposition á when you say what someone is listening to.

So you learn it as:

• hlusta á = listen to

Examples:

• hlusta á tónlist = listen to music
• hlusta á útvarpið = listen to the radio
• hlusta á mig = listen to me

This is a good example of something that should be learned as a set expression: not just the verb hlusta, but hlusta á.

Yes. In this sentence, á goes with the accusative because it belongs to the verb phrase hlusta á.

So tónlist is functioning as the object of the preposition.

For the noun tónlist, the singular form is the same in nominative and accusative, so you do not see a visible change here:

• nominative: tónlist
• accusative: tónlist

That is why the form looks unchanged.

Because tónlist is being used in a general sense: music in general, not a specific piece or set of music.

So:

• hlusta á tónlist = listen to music

If you wanted to mean the music, you could say:

• hlusta á tónlistina = listen to the music

Icelandic often leaves nouns without the definite article when talking about things in a general way.

Most naturally, it expresses a general habit or something that usually happens:

• I fall asleep quickly when I listen to music

The present tense in Icelandic, like in English, can be used for habitual actions, not only for what is happening at this exact moment.

So this sentence usually means something like Whenever I listen to music, I fall asleep quickly.

The sentence has:

1. a main clause
   
   • Ég sofna fljótt
2. a subordinate clause introduced by þegar
   
   • þegar ég hlusta á tónlist

So the full structure is:

• Ég = subject
• sofna = verb
• fljótt = adverb
• þegar ég hlusta á tónlist = time clause

A useful comparison is this alternative order:

• Þegar ég hlusta á tónlist, sofna ég fljótt.

That also means the same thing, but because the subordinate clause comes first, the main-clause verb sofna comes before the subject ég. This is part of Icelandic’s verb-second pattern in main clauses.

Not if you want the same meaning.

• Ég sofna fljótt = I fall asleep quickly
• Ég sef fljótt would not be the normal way to say I sleep quickly, and that does not really express the same idea anyway.

If the point is that music makes you drift off fast, sofna is the correct verb.