Το ντουέτο τελειώνει.

A natural pronunciation is:

to du-É-to te-li-Ó-ni

A few helpful sound notes:

• το = to
• ντ at the beginning of ντουέτο usually sounds like d
• ει and οι are both pronounced i in Modern Greek
• The accent marks show the stressed syllable:
  • ντουέτο → stress on έ
  • τελειώνει → stress on ώ

So the whole sentence sounds roughly like to du-É-to te-li-Ó-ni.

το is the definite article, meaning the.

In this sentence, it is:

• neuter
• singular
• nominative (because it goes with the subject of the sentence)

So το ντουέτο means the duo or the duet.

ντουέτο is a neuter noun in Greek, so it takes the neuter article το.

This is very common with loanwords ending in -ο.
Since ντουέτο is a borrowed word, Greek treats it as neuter:

• το ντουέτο

This is something you usually just learn together with the noun: not just ντουέτο, but το ντουέτο.

It can correspond to duo or duet, depending on context.

In Greek, ντουέτο is used for:

• two performers together
• a musical duet
• sometimes more generally, a pair acting together

So the exact English translation depends on the situation, but grammatically it works the same way in this sentence.

τελειώνει is the 3rd person singular present tense form of the verb τελειώνω, which means to finish or to end.

So:

• τελειώνω = I finish / I am finishing
• τελειώνει = he/she/it finishes, ends / is finishing, is ending

Here it agrees with το ντουέτο, which is singular, so Greek uses τελειώνει.

It can mean either one.

The Greek present tense often covers both:

• simple present: ends
• progressive present: is ending

So Το ντουέτο τελειώνει could mean:

• The duo ends
• The duo is ending
• The duet is ending
• The duet finishes

The exact choice in English depends on context.

Greek usually does not need an explicit subject pronoun when the subject is clear.

Since the sentence already has the noun το ντουέτο, there is no need to add a word for it.

Also, the verb ending -ει already tells you it is 3rd person singular.

So Greek naturally says:

• Το ντουέτο τελειώνει

not something like:

• Το ντουέτο αυτό τελειώνει unless you specifically want emphasis like this duo is ending

Yes. Greek word order is more flexible than English.

The neutral order here is:

• Το ντουέτο τελειώνει.

But you could also say:

• Τελειώνει το ντουέτο.

That version is possible, but it changes the emphasis or flow a bit.
Very roughly:

• Το ντουέτο τελειώνει = neutral statement
• Τελειώνει το ντουέτο = more focus on ending or on what is ending

So the original order is the most straightforward one for learners.

The accent marks show which syllable is stressed.

• ντουέτο → stress on έ
• τελειώνει → stress on ώ

Stress matters in Greek pronunciation, so it is important to learn words with their accent marks.

Without paying attention to stress, your Greek may still be understandable, but it will sound much less natural.

In this sentence, τελειώνει is used intransitively, meaning:

• comes to an end
• is ending

So the duo is the thing that ends.

In other contexts, forms of τελειώνω can also be used in ways that relate to finishing something, but here the meaning is clearly:

• The duo/duet ends
• The duo/duet is ending

So for this sentence, it is best to understand it simply as to end / to be ending.

Yes. It is short, grammatical, and natural.

It sounds like a normal standalone sentence meaning that the duo or duet is coming to an end. Greek often allows very short complete sentences like this, especially when the context is already known.