Questa decisione coinvolge tutti.

Because decisione is a feminine singular noun, and the demonstrative has to agree with it.

• questo = this, for masculine singular nouns
• questa = this, for feminine singular nouns

So:

• questo libro = this book
• questa decisione = this decision

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

In Italian, you normally do not use an article together with questo/questa:

• questa decisione = this decision
  not la questa decisione

This is the same idea as English: you say this decision, not the this decision.

Coinvolge is the third-person singular present tense of coinvolgere.

So the pattern is:

• io coinvolgo = I involve
• tu coinvolgi = you involve
• lui/lei coinvolge = he/she involves

Here the subject is questa decisione, which is singular, so Italian uses the singular verb form coinvolge.

Because the verb agrees with the subject, not with the object.

In this sentence:

• Questa decisione = subject
• coinvolge = verb
• tutti = direct object

Since questa decisione is singular, the verb must be singular: coinvolge.

English works the same way:

• This decision involves everyone not This decision involve everyone

Here tutti means everyone or all people concerned, depending on context.

Literally, tutti means all or everyone. In this sentence, it is being used on its own, so it means something like:

• everyone
• everybody
• all of them

The exact English translation can vary a little depending on context, but grammatically tutti is the plural object of the verb.

In Italian, the masculine plural is also used for a mixed group or for a group whose gender is not being specified.

So:

• tutti = everyone / all of them / all (mixed group or unspecified)
• tutte = all of them, if the group is entirely feminine

That means tutti is the normal choice unless you specifically mean only women or feminine nouns.

Only if the people or things involved are all feminine.

For example:

• Questa decisione coinvolge tutte le studentesse. = This decision involves all the female students.

But if you mean everyone in general, tutti is the standard form.

Because coinvolgere takes a direct object in Italian.

So you say:

• coinvolgere qualcuno = to involve someone

not

• coinvolgere a qualcuno

This is good news for English speakers, because it behaves a lot like English to involve someone.

Yes. Tutti is the direct object.

A quick way to see it is to ask:

• What is doing the action? → questa decisione
• Whom does it involve? → tutti

So questa decisione is the subject, and tutti receives the action.

This is the most neutral and natural word order in ordinary Italian:

• subject + verb + object

Italian word order is somewhat flexible, but changing it usually adds emphasis or sounds more literary, dramatic, or context-dependent.

For example:

• Coinvolge tutti, questa decisione.

This is possible, but it is not the plainest default version. For most learners, Questa decisione coinvolge tutti is the safest and most natural choice.

The infinitive is coinvolgere.

It belongs to the -ere group. Its present tense forms are:

• io coinvolgo
• tu coinvolgi
• lui/lei coinvolge
• noi coinvolgiamo
• voi coinvolgete
• loro coinvolgono

It is fairly straightforward to learn as a verb form, though like many Italian verbs, some forms are better learned by exposure and practice rather than by trying to force a perfect rule from English.

A helpful approximation is:

coin-VOL-jeh

A few details:

• coin sounds roughly like koin
• the stress is on VOL
• ge sounds like the j in jam

So the whole word is approximately koin-VOL-jeh.

A useful approximation is:

deh-chee-ZYO-neh

A few points:

• the stress is on ZYO
• ci here sounds like ch in cheese
• the s sounds like a z sound in this word

So: deh-chee-ZYO-neh

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

Coinvolgere can mean:

• to involve
• to include in something
• to affect directly
• sometimes even to engage emotionally

So in context, Questa decisione coinvolge tutti could mean:

• this decision involves everyone
• this decision affects everyone
• this decision concerns everyone

The best English choice depends on context, even though involves everyone is a very natural translation.

Yes, but the meaning would shift slightly.

For example:

• Questa decisione riguarda tutti. = This decision concerns everyone.
• Questa decisione interessa tutti. = This decision interests everyone / is relevant to everyone.
• Questa decisione tocca tutti. = This decision affects everyone.

Coinvolge suggests that everyone is somehow drawn into or impacted by the decision, not just that it is vaguely about them.

They are related, but they are not the same form.

• tutto = all / everything / whole, masculine singular
• tutti = all / everyone / all of them, masculine plural

Examples:

• tutto il giorno = the whole day
• tutto = everything
• tutti = everyone / all of them

So in your sentence, tutti is used because it refers to all people rather than everything or a whole singular thing.