Praticamente tutti hanno già pagato.

Here praticamente means something like practically, almost, or more or less in the sense of nearly everyone.

So Praticamente tutti hanno già pagato means that almost everyone has already paid, not necessarily every single person.

Be careful: praticamente can also mean basically in other contexts, especially in spoken Italian. So it is often a close cousin of English practically, but the exact nuance depends on the sentence.

In this sentence, it clearly modifies tutti:

• praticamente tutti = practically everyone

Because tutti is plural, while tutto is singular.

• tutto = everything / all of it
• tutti = everyone / all of them / all

In this sentence, the idea is all the people or everyone, so the plural form tutti is needed.

Compare:

• Tutto è pronto. = Everything is ready.
• Tutti sono pronti. = Everyone is ready. / They are all ready.

It works like everyone here.

In Italian, tutti can be:

• an adjective: tutti i libri = all the books
• a pronoun: tutti hanno pagato = everyone has paid / they have all paid

In your sentence, tutti is being used on its own, so it is a pronoun.

Because tutti refers to more than one person grammatically, so the verb must be plural.

• tutti hanno = everyone has / all have
• not tutti ha

Even though English often uses everyone has with a singular verb, Italian treats tutti as plural, so you use hanno.

Italian usually drops subject pronouns when the verb ending already makes the subject clear.

• hanno already tells you the subject is they
• so loro is unnecessary in most cases

That is why Tutti hanno già pagato sounds natural, while Loro hanno già pagato would usually only be used for emphasis or contrast.

It is the passato prossimo, one of the main past tenses in Italian.

It is formed with:

• present tense of the auxiliary verb
• past participle

Here:

• hanno = present of avere
• pagato = past participle of pagare

So:

• hanno pagato = have paid / paid

In many everyday situations, Italian uses passato prossimo where English might use either have paid or paid, depending on context.

Because with avere, the past participle usually does not agree with the subject.

So you say:

• tutti hanno pagato
• Maria ha pagato
• le ragazze hanno pagato

The participle stays pagato.

Agreement usually happens with essere:

• sono arrivati
• Maria è arrivata

With avere, agreement can sometimes happen with a preceding direct object pronoun, but that is a more advanced point and does not apply here.

Già means already.

In hanno già pagato, it comes between the auxiliary and the past participle:

• hanno già pagato

This is a very common and natural position for adverbs in compound tenses.

You may also hear different placements in some contexts, but this is the standard one and a very good model to follow.

Compare:

• Ho già mangiato. = I have already eaten.
• Abbiamo già finito. = We have already finished.

Yes. Quasi tutti hanno già pagato is also very natural and often very close in meaning.

Difference in nuance:

• quasi tutti = almost everyone
• praticamente tutti = practically everyone, nearly everyone, more or less everyone

Praticamente tutti can sound a little more conversational or approximate, while quasi tutti can sound slightly more straightforward and literal. In many contexts, though, they are interchangeable.

Tutti can refer to a mixed group or to people in general.

Use:

• tutti for a mixed group or when gender is unspecified
• tutte only for an all-female group

So:

• Praticamente tutti hanno già pagato. = practically everyone has already paid
• Praticamente tutte hanno già pagato. = practically all of them have already paid, if the group is all female

Yes, but the given order is the most neutral and natural one.

Standard:

• Praticamente tutti hanno già pagato.

Possible alternatives for emphasis:

• Hanno già pagato praticamente tutti.
• Già, praticamente tutti hanno pagato. (in a suitable context)

These can be correct, but they may sound more marked or depend more on context. For a learner, Praticamente tutti hanno già pagato is the safest and most natural model.

It is neutral and very natural in everyday Italian.

Nothing in it is especially formal or especially slangy.
The only thing to note is that praticamente is very common in speech and informal writing, but it is also perfectly acceptable in normal standard Italian.

So this sentence would sound fine in conversation, emails, and most ordinary contexts.

No, that would usually mean something different or sound unnatural here.

• hanno pagato = they have paid
• sono pagati = they are paid

The original sentence talks about the action performed by the people: they paid.

If you said sono pagati, you would be talking about their state of being paid, which usually does not fit with pagare in this context.

So for they have already paid, you need:

• hanno già pagato