Heyrnartólin mín virka vel, en hátalarinn í tölvunni virkar ekki vel.

Because heyrnartól means headphones, and in Icelandic it is treated as a plural item, much like English headphones. The ending -in is the suffixed definite article, so heyrnartólin means the headphones.

That is also why the verb is plural: heyrnartólin mín virka = my headphones work.

In Icelandic, possessive words like minn, mín, mitt often come after the noun in ordinary, neutral phrasing.

So:

• heyrnartólin mín = my headphones

This is a very common pattern. Putting the possessive first can be possible in some contexts, but it often sounds more emphatic or marked.

Because the possessive has to agree with the noun in gender, number, and case.

Heyrnartólin is:

• neuter
• plural
• nominative

The matching form of minn is therefore mín.

Very roughly:

• masculine plural: mínir
• feminine plural: mínar
• neuter plural: mín

So heyrnartólin mín is grammatically correct because heyrnartólin is neuter plural.

The verb agrees with the subject in number.

• Heyrnartólin mín is plural, so the verb is virka
• Hátalarinn is singular, so the verb is virkar

So:

• heyrnartólin mín virka = my headphones work
• hátalarinn virkar = the speaker works

It is the definite article attached to the end of the noun.

• hátalari = speaker
• hátalarinn = the speaker

This is very common in Icelandic. Instead of using a separate word like English the, Icelandic often adds the article directly onto the noun.

Because í can take different cases depending on meaning.

Here it expresses location, so it takes the dative:

• í tölvu = in a computer
• í tölvunni = in the computer

The -nni part includes the definite article and the dative ending.

If í expresses motion into something, it often takes the accusative instead. But here the meaning is location, not movement.

Yes. Icelandic uses í for something physically inside something else.

So hátalarinn í tölvunni naturally means the speaker in the computer, which suggests a built-in speaker, not an external speaker sitting next to it.

Because each clause has its own verb phrase, and vel belongs with each one separately.

• heyrnartólin mín virka vel
• hátalarinn í tölvunni virkar ekki vel

Repeating vel makes the contrast clear:

• the headphones work well
• the computer speaker does not work well

It would sound less complete if the second vel were left out here.

Icelandic often forms long compound words.

Here are the main ones:

• heyrnartólin
  = heyrnar

  • tól
    • -in
      roughly hearing-device(s)-the
      meaning the headphones

• hátalarinn
  = hátalari

  • -inn
    meaning the speaker

• tölvunni
  = tölvu

  • definite/dative ending
    meaning in the computer

Learning to spot compounds and endings is a big part of reading Icelandic.

No. In Icelandic, they are separate letters, not just stress marks.

So:

• á
• í
• ó

represent different vowel sounds from:

• a
• i
• o

That means the accents are part of the spelling and pronunciation, not optional marks.

Because en is joining two full clauses here:

• Heyrnartólin mín virka vel
• hátalarinn í tölvunni virkar ekki vel

Using a comma before en is normal in Icelandic in a sentence like this.

Yes. Virkar illa means works badly, and it is a perfectly natural alternative.

There is a slight difference in tone:

• ekki vel = not well, a bit softer
• illa = badly, a bit more direct

So this sentence could also be said as:

Heyrnartólin mín virka vel, en hátalarinn í tölvunni virkar illa.