Tónleikurinn byrjar seint í kvöld.

The ending -inn is the definite article attached to the noun.

• tónleikur = concert
• tónleikurinn = the concert

In Icelandic, the is usually not a separate word like it is in English. Instead, it is often added to the end of the noun.

It is in the nominative singular because it is the subject of the sentence: it is the thing that starts.

So here:

• Tónleikurinn = the subject
• byrjar = the verb

That is why you see the nominative form.

Yes. Tónleikur is a valid singular noun meaning concert, and tónleikurinn means the concert.

You will also very often see tónleikar, which is a very common word for concerts / a concert performance. Icelandic sometimes uses plural forms where English prefers a singular. So this sentence uses a perfectly normal singular form, but learners should also be aware that tónleikar is very common in real life.

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

Because the subject is singular:

• ég byrja = I start
• þú byrjar = you start
• hann/hún/það byrjar = he/she/it starts

So Tónleikurinn byrjar means the concert starts.

Here seint is being used as an adverb, meaning late.

The adjective is:

• seinn = late, masculine
• sein = late, feminine
• seint = late, neuter

In Icelandic, the neuter singular form is often used as an adverb. So:

• Hann er seinn = He is late
• Hún er sein = She is late
• Það er seint = It is late
• Hann kemur seint = He comes late

In your sentence, seint describes how the concert starts, so the adverb form is needed.

Í kvöld is a fixed time expression meaning tonight or this evening.

Literally, it is:

• í = in
• kvöld = evening

But as a whole, í kvöld is the normal Icelandic way to say tonight.

After í, Icelandic can use different cases depending on meaning. In this expression, í kvöld uses the accusative.

The reason you do not see a visible change is that kvöld is a neuter singular noun, and in this case the accusative form looks the same as the basic dictionary form.

So the case is there grammatically, but it is not obvious from the spelling.

This is normal Icelandic main-clause word order:

• Tónleikurinn = subject
• byrjar = finite verb
• seint í kvöld = adverbial information

A very important rule in Icelandic is that the finite verb usually comes second in a main clause. Here the subject comes first, so the verb naturally comes second.

Yes. You can say:

Í kvöld byrjar tónleikurinn seint.

That is also correct. It puts more emphasis on tonight.

Notice what happens: once Í kvöld moves to the first position, the finite verb byrjar still stays in the second position, and the subject tónleikurinn moves after it. That is a classic Icelandic verb-second pattern.

A rough learner-friendly pronunciation might be:

TOHN-lay-kur-rin BIR-yar saynt ee KVOLT

A few helpful notes:

• ó is a long vowel, roughly like oh
• ei sounds roughly like ay
• y is pronounced like Icelandic i, not like English y
• í is a long ee
• kv is pronounced clearly as a cluster

If you want a more IPA-style version, a rough guide is:

[ˈtʰouːnˌleiːkʏrɪn ˈpɪrjar seint iː ˈkʰvœlt]

Exact pronunciation varies a bit by speaker.

Not if you want the meaning starts late tonight.

• byrjar = starts / begins
• er = is

So Tónleikurinn byrjar seint í kvöld means the concert’s starting time is late.

If you said Tónleikurinn er í kvöld, that would mean The concert is tonight, which is a different sentence and gives less specific information.

You can say:

Tónleikurinn byrjar ekki seint í kvöld.

Here ekki means not.

In a simple main clause like this, ekki usually comes after the finite verb:

• byrjar ekki = does not start

So the sentence means The concert does not start late tonight.