Repetiția începe azi.

Because -a at the end is the definite article attached to the noun.

In Romanian, instead of putting a separate word like the before the noun, you usually add the article to the end:

• repetiție = a rehearsal
• repetiția = the rehearsal

So Repetiția începe azi. literally means The rehearsal starts today.

Repetiție is a feminine noun. That matters because the form of the attached definite article depends on gender and number.

For this noun:

• singular indefinite: o repetiție = a rehearsal
• singular definite: repetiția = the rehearsal

So the -a ending here is the normal feminine singular definite ending.

Începe is the 3rd person singular present tense of the verb a începe, which means to begin / to start.

So:

• eu încep = I start
• tu începi = you start
• el/ea începe = he/she/it starts
• repetiția începe = the rehearsal starts

The subject here is repetiția, which is singular, so începe is the correct form.

Romanian often does not use subject pronouns unless they are needed for emphasis or clarity.

In English, you must say:

• It starts today

But in Romanian, the verb ending already helps show the subject, and if the noun is present, no pronoun is needed:

• Repetiția începe azi.

You would not normally add a separate word meaning it here.

Yes. Azi and astăzi both mean today.

• azi = shorter, very common, natural in everyday speech
• astăzi = also very common, sometimes felt as slightly fuller or a bit more formal

So both are correct:

• Repetiția începe azi.
• Repetiția începe astăzi.

Yes. Romanian word order is fairly flexible, although the most neutral version here is:

• Repetiția începe azi.

You can also say:

• Azi începe repetiția.

That version puts more emphasis on today. It is still completely natural.

So:

• Repetiția începe azi. = neutral statement
• Azi începe repetiția. = emphasis on today

A simple learner-friendly pronunciation guide is:

• Repetiția ≈ reh-peh-tee-TSI-ah
• începe ≈ in-cheh-peh, with Romanian î pronounced as a close central sound that English does not really have
• azi ≈ ah-ZEE

A few important pronunciation points:

• ț sounds like ts in cats
• ce sounds like che in check
• î is a special Romanian vowel, not exactly like any standard English vowel

The main stresses are:

• repeTIția
• înCEpe
• aZI

There are two important Romanian diacritics here:

• ț in repetiția
  This is pronounced ts.

• î in începe
  This is a special Romanian vowel sound.

Diacritics are important because they can change pronunciation and sometimes meaning. In careful writing, you should include them:

• correct: Repetiția începe azi.
• not recommended in formal writing: Repetitia incepe azi.

You may sometimes see text without diacritics online, but learners should get used to the correct forms.

It can mean either begins or starts. In this sentence, both are good translations:

• The rehearsal begins today.
• The rehearsal starts today.

In everyday English, starts often sounds a bit more natural and conversational, while begins can sound slightly more formal. Romanian a începe covers both ideas.

Normally, no. The natural sentence is:

• Repetiția începe azi.

Adding se here would sound unnatural for this meaning. Romanian often uses se in reflexive or impersonal constructions, but for a straightforward sentence like The rehearsal starts today, the simple verb începe is the normal choice.

No. Azi can appear in different positions, depending on emphasis.

Common options:

• Repetiția începe azi.
• Azi începe repetiția.
• Repetiția azi începe. — possible in special contexts, but less neutral

For most learners, the first two are the most useful and natural.