Questa fase inizia domani.

Because fase is a feminine singular noun in Italian, and the demonstrative adjective has to agree with it.

• questo = this (masculine singular)
• questa = this (feminine singular)

So:

• questo libro = this book
• questa fase = this phase

In Italian, every noun has a grammatical gender, and fase is feminine. You usually learn the gender together with the noun.

A useful clue is the article:

• la fase = the phase

Since it takes la, it is feminine.
Not all feminine nouns end in -a, so fase is a good reminder that gender is not always obvious from the ending alone.

Because questa already functions as a determiner, like this in English.

In English, you say:

• this phase not
• the this phase

Italian works the same way:

• questa fase not
• la questa fase

So questa replaces the article.

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

Conjugation:

• io inizio = I begin
• tu inizi = you begin
• lui/lei inizia = he/she/it begins
• noi iniziamo = we begin
• voi iniziate = you all begin
• loro iniziano = they begin

In Questa fase inizia domani, the subject is questa fase, which is singular, so inizia is the correct form.

Italian often leaves out subject pronouns when they are not needed, because the verb ending already gives useful information.

So instead of saying:

• Questa fase essa inizia domani

Italian simply says:

• Questa fase inizia domani

This is completely normal. In fact, adding a subject pronoun here would usually sound unnecessary or unnatural.

It can mean either, depending on context.

The Italian present tense is often broader than the English simple present. It can express:

• a general present: begins
• a near future: is beginning / starts
• sometimes an ongoing present, depending on context

In this sentence, because of domani (tomorrow), the meaning is clearly future-related:

• This phase begins tomorrow or
• This phase starts tomorrow

Italian very often uses the present tense for future events, especially when there is a clear time expression like:

• domani = tomorrow
• stasera = tonight
• la prossima settimana = next week

So inizia domani is perfectly natural Italian.

Italian could also use a future form:

• Questa fase inizierà domani

That is also correct, but the present tense is very common and natural in everyday speech.

Yes. Iniziare and cominciare both mean to begin / to start and are often interchangeable.

So both are correct:

• Questa fase inizia domani
• Questa fase comincia domani

Sometimes one may sound slightly more natural than the other depending on context or personal preference, but both are standard.

Yes, Italian word order is fairly flexible, though the original version is the most neutral:

• Questa fase inizia domani.

You could also say:

• Domani inizia questa fase.

That version puts more focus on domani or presents the timing first.

So the word order can change, but it may slightly change emphasis.

A rough pronunciation guide is:

• QUE-sta FA-ze i-NI-tsya do-MA-ni

A few helpful points:

• qu sounds like kw
• s in fase sounds like a voiced z in standard pronunciation
• zia in inizia sounds roughly like tsya
• stress falls on:
  • QUEsta
  • FAse
  • iniZIA
  • doMAni

No. Domani can appear in different positions.

All of these are possible:

• Questa fase inizia domani.
• Domani questa fase inizia.
• Domani inizia questa fase.

The most common and neutral choice here is the original sentence, with domani at the end. Moving it changes the emphasis more than the basic meaning.

Sometimes, yes. Fase often corresponds directly to phase, but in some contexts English might prefer stage.

For example, depending on the situation:

• questa fase = this phase
• questa fase del progetto = this stage/phase of the project

So the best English word can depend on context, even though the Italian word stays the same.

It is usually learned as part of the demonstrative set based on questo (this), which changes for gender and number:

• questo = this (masculine singular)
• questa = this (feminine singular)
• questi = these (masculine plural)
• queste = these (feminine plural)

So questa is the feminine singular form of the demonstrative adjective meaning this.