Дуэт начинается сегодня.

Дуэт is in the nominative singular.

Why:

• It is the subject of the sentence — the thing that is doing the action of beginning.
• In Russian, the subject of a normal sentence is usually in the nominative case.

So in Дуэт начинается сегодня, дуэт = the duet / the duo as the subject.

It is also a masculine noun, because it ends in a consonant.

Начинается comes from начинаться, which means to begin / to start.

The ending -ся is important here. It often makes a verb:

• reflexive,
• or more loosely, intransitive / centered on the event happening itself.

Compare:

• начинать что-то = to begin something
• начинаться = to begin, to start

So:

• Оркестр начинает концерт. = The orchestra begins the concert.
• Концерт начинается. = The concert is beginning / begins.

In your sentence, the duet is not beginning something else; the duet itself is starting. So начинается is the natural form.

In Russian, the present tense of an imperfective verb can be used for:

• something happening now,
• something regularly happening,
• or something scheduled / about to happen.

So Дуэт начинается сегодня can mean:

• The duet starts today
• The duet is beginning today

Russian often uses the present tense this way, especially for schedules, events, and planned actions.

This is mainly a question of aspect.

• начинается = imperfective
• начнётся = perfective

Very roughly:

• Дуэт начинается сегодня
  focuses on the event as something scheduled or in progress as it starts.

• Дуэт начнётся сегодня
  focuses more on the single completed starting point: it will start today.

In many everyday contexts, both can work, but the nuance is slightly different.

A simple way to remember it:

• начинается = is starting / starts
• начнётся = will start

A helpful pronunciation guide is:

• дуэ́т = doo-ET
• начина́ется = na-chee-NA-ye-tsa
• сего́дня = roughly si-VOD-nya

A few important stress points:

• дуэ́т — stress on the last syllable
• начина́ется — stress on на́
• сего́дня — stress on го́

A useful extra note: сегодня is spelled with г, but in normal pronunciation it sounds more like в in this word.

Yes. Сегодня is an adverb of time meaning today.

It tells you when the duet begins.

Because it is an adverb, it does not change form here. You do not need to decline it in this sentence.

Yes. Russian word order is more flexible than English word order.

Your sentence has a very neutral structure:

• Дуэт начинается сегодня.

But you could also say:

• Сегодня начинается дуэт.
• Дуэт сегодня начинается.

These all mean roughly the same thing, but the emphasis changes a little:

• Дуэт начинается сегодня. = neutral
• Сегодня начинается дуэт. = stronger focus on today
• Дуэт сегодня начинается. = slight emphasis on the timing, often depending on context

Russian word order often helps show focus or what is new information.

In Russian, a noun that ends in a hard consonant is very often masculine.

So дуэт is masculine.

That matters for agreement. For example:

• новый дуэт = a new duet
• известный дуэт = a famous duet

And in the past tense:

• Дуэт начался. = The duet began.

The masculine past tense ending there is -ся after начал → начался.

Russian does not have articles like a, an, or the.

So дуэт can mean:

• a duet
• the duet

Which one is meant depends on:

• context,
• previous mention,
• and the situation.

In a standalone sentence, English might choose the duet or a duet depending on context, but Russian simply says дуэт.

Not always.

Дуэт can refer to:

• a duet in music,
• or a duo / pair of performers.

So depending on context, Дуэт начинается сегодня could mean:

• a duet performance starts today,
• or a performing duo begins today, if the context supports that reading.

Usually the surrounding context makes the intended meaning clear.