La selezione inizia domani mattina alle nove.

La is the definite article, meaning the.

So la selezione = the selection.

In Italian, articles are often used more regularly than in English. In a sentence like this, if you are talking about a specific event or process, Italian normally includes the article.

• la = the for a singular feminine noun
• selezione is feminine singular, so it takes la

Selezione literally means selection, but depending on context it could refer to things like:

• a selection process
• tryouts
• an audition
• a recruitment round
• a screening

So the exact English translation depends on the situation, but grammatically it is just a normal feminine noun meaning selection.

Inizia is the third person singular present tense of iniziare (to begin, to start).

Here it agrees with la selezione, which is singular:

• la selezione inizia = the selection begins / starts

Conjugation of iniziare in the present:

• io inizio
• tu inizi
• lui/lei inizia
• noi iniziamo
• voi iniziate
• loro iniziano

Italian often uses the present tense to talk about scheduled future events, just like English can do:

• The meeting starts tomorrow
• La riunione inizia domani

So inizia domani mattina is completely natural Italian. You do not need a special future tense here.

You could also use the future:

• La selezione inizierà domani mattina alle nove

But the present is very common and natural for plans, schedules, and timetables.

Domani mattina is the normal, natural way to say tomorrow morning.

Italian often uses time expressions in a fixed, simple way:

• domani mattina = tomorrow morning
• domani sera = tomorrow evening
• ieri mattina = yesterday morning
• stamattina = this morning

La mattina di domani is grammatically possible in some special contexts, but it sounds much less natural for everyday speech.

Alle nove means at nine o’clock.

Alle is a combination of:

• a = at / to
• le = the

So literally it is something like at the nine, though you should think of it simply as the normal Italian way to tell time.

Examples:

• all’una = at one
• alle due = at two
• alle nove = at nine

Notice the difference:

• all’una for one o’clock
• alle
  • plural hour for most other hours

Italian usually does not need a separate word like o’clock.

So:

• alle nove = at nine / at nine o’clock

If needed, Italian can say nove in punto for exactly nine o’clock, but normally alle nove is enough.

That order is very natural in Italian: day/time period + exact time.

So:

• domani mattina = tomorrow morning
• alle nove = at nine

Together:

• domani mattina alle nove = tomorrow morning at nine

Italian word order is often flexible, but this order sounds smooth and standard.

You could also say:

• La selezione inizia alle nove domani mattina

but La selezione inizia domani mattina alle nove sounds more natural in neutral speech.

A simple pronunciation guide:

• selezione ≈ seh-leht-TSYOH-neh
• inizia ≈ ee-NEE-tsya

A few helpful points:

• z in selezione is pronounced like ts
• -zione is a very common ending, pronounced roughly -tsyoh-neh
• inizia has the stress on ni: i-NI-zia

Yes. Comincia and inizia can both mean starts/begins.

So you could say:

• La selezione comincia domani mattina alle nove.

That is also correct and natural.

In many situations, iniziare and cominciare are interchangeable, though iniziare can sometimes sound slightly more formal depending on context.

It is neutral and works in both formal and informal contexts.

Nothing in the sentence is especially casual or especially formal. It is just a standard factual statement:

• La selezione inizia domani mattina alle nove.

That makes it suitable for speech, announcements, emails, notices, and general conversation.