Questa azione cambia tutto.

Because azione is a feminine singular noun, and the demonstrative must agree with it.

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

So:

• questa azione = this action

Even though azione ends in -e, it is still feminine.

You usually learn the gender with the noun, but there is also a useful pattern:

• nouns ending in -zione are almost always feminine

So:

• l’azione = the action
• questa azione = this action

This is a very common pattern in Italian, like:

• nazione
• informazione
• situazione

They are all feminine.

Cambia is the third person singular present form of cambiare.

The subject is questa azione, which is singular, so the verb must also be singular:

• questa azione cambia = this action changes

Here is the present tense of cambiare:

• io cambio = I change
• tu cambi = you change
• lui/lei cambia = he/she changes
• noi cambiamo = we change
• voi cambiate = you all change
• loro cambiano = they change

It can mean either, depending on context.

Italian present tense often covers both:

• changes
• is changing

So Questa azione cambia tutto could mean:

• This action changes everything
• This action is changing everything

Without more context, English usually translates it as changes everything.

Because questa already works like this, and in Italian demonstratives usually replace the article.

So you say:

• questa azione = this action

not normally:

• la questa azione ✘

This is similar to English, where you say this action, not the this action.

Here, tutto means everything.

In this sentence it is being used on its own, almost like a pronoun:

• cambia tutto = changes everything

Be careful: tutto can also mean all, whole, or entire, depending on the sentence.

Examples:

• tutto il giorno = the whole day
• tutti gli studenti = all the students
• tutto = everything

Because here the meaning is everything, not all people or all things as a plural group.

• tutto = everything
• tutti = everyone / all of them / all (masculine plural)
• tutte = all (feminine plural)

So:

• Questa azione cambia tutto = This action changes everything

If you said cambia tutti, it would usually mean changes everyone or changes them all, which is a different idea.

Yes. Italian normally does not need a separate subject pronoun if the subject is already stated or understood.

In this sentence, the subject is explicitly given:

• Questa azione = this action

So there is no need for an extra pronoun like essa or lei.

Italian would not normally say something like:

• Questa azione essa cambia tutto ✘

That would sound unnecessary or wrong in normal usage.

The most natural and neutral order is:

• Questa azione cambia tutto.

That is:

• subject + verb + object

Italian word order is somewhat flexible, but changing it changes the emphasis.

For example:

• Tutto cambia con questa azione. = Everything changes with this action.
• Con questa azione, tutto cambia. = With this action, everything changes.

These are not exactly the same in focus as the original sentence.

Yes, grammatically you could, and it means something very similar.

• cambia tutto = changes everything
• cambia ogni cosa = changes every single thing

But cambia tutto is more natural and common in everyday Italian.

Ogni cosa can sound a little more emphatic, literary, or deliberate depending on context.

A simple pronunciation guide is:

KWEH-sta at-TSYO-ne KAM-bya TOOT-to

A few helpful notes:

• qu in questa sounds like kw
• zi in azione sounds like tsy here
• gli does not appear here, so pronunciation is fairly straightforward
• tt in tutto is pronounced as a stronger, longer t

If you say it smoothly:

KWEH-sta at-TSYO-ne KAM-bya TOOT-to

Often yes, but not always in exactly the same way as English action.

Depending on context, azione can mean:

• action
• act
• move
• step
• operation

So in some contexts, Questa azione cambia tutto might also be understood as:

• This move changes everything
• This act changes everything

But if your given meaning is This action changes everything, then azione is a perfectly normal choice.