Trommarinn brosir þegar fyrsta lagið byrjar.

Because Icelandic usually puts the definite article on the end of the noun instead of using a separate word like English the.

• trommari = drummer
• trommarinn = the drummer

In this sentence, trommarinn is in the nominative singular because it is the subject of brosir.

Brosir is the present tense, 3rd person singular form of the verb brosa, meaning to smile.

So:

• ég brosi = I smile
• þú brosir = you smile
• hann/hún/það brosir = he/she/it smiles

Here the subject is trommarinn, which is singular, so brosir is the correct form.

Here þegar means when.

It introduces a time clause:

• Trommarinn brosir = the drummer smiles
• þegar fyrsta lagið byrjar = when the first song starts

So þegar connects the two parts of the sentence.

A learner should also know that þegar can sometimes mean already in other contexts, but in this sentence it is clearly the conjunction when.

Because lag is a neuter noun, and the adjective has to agree with it.

The adjective here is fyrsti / fyrsta / fyrsta = first, and its form changes depending on gender, number, case, and whether the noun is definite.

Since lagið is:

• singular
• neuter
• definite

the correct form is fyrsta.

So:

• fyrsti would be a masculine form
• fyrsta is the correct neuter form here

Because lagið means the song, while lag means just song.

Again, Icelandic usually adds the definite article to the end of the noun:

• lag = song
• lagið = the song

So fyrsta lagið means the first song, not just first song.

It is nominative, because it is the subject of byrjar in the clause þegar fyrsta lagið byrjar.

In other words, the first song is the thing that is doing the action of starting.

A useful extra point: for many neuter nouns in the singular, the nominative and accusative forms look the same, so the role in the sentence helps you identify the case.

Byrjar is the present tense, 3rd person singular of byrja, meaning to begin or to start.

So here:

• fyrsta lagið byrjar = the first song starts / begins

Like brosir, it is 3rd person singular because the subject, fyrsta lagið, is singular.

Yes. You can also put the þegar clause first:

Þegar fyrsta lagið byrjar, brosir trommarinn.

That still means the same thing.

This is useful because Icelandic often allows flexible word order, but the verb placement matters. In the version with the time clause first, the main clause still keeps the finite verb early:

• Þegar fyrsta lagið byrjar, brosir trommarinn

Not Þegar fyrsta lagið byrjar, trommarinn brosir in standard Icelandic.

Two letters often stand out to English speakers:

• þ in þegar is like the th in thin
• ð in lagið is like the th in this, though in some positions it can sound weaker

A rough pronunciation guide for the whole sentence could be:

TROM-ma-rin BRO-sir THYE-gar FIR-sta LA-yið BYR-yar

A few extra notes:

• j in Icelandic is pronounced like English y
• so þegar begins with a th sound, not a t
• byrjar has a y sound that English speakers may need practice with

Yes. Icelandic commonly expresses this idea with:

• an adjective
• plus a noun with the suffixed definite article

So fyrsta lagið is the normal way to say the first song.

This may feel different from English because English puts the before the whole noun phrase, while Icelandic often marks definiteness directly on the noun:

• English: the first song
• Icelandic: fyrsta lagið

The adjective agrees with the noun, but the definite article is attached to the noun itself.