Tutti arrivano tranne Marco.

Because tutti is the masculine plural form and here it means everyone / all (the people).

A quick comparison:

• tutti = all, everyone, all of them (masculine plural or mixed group)
• tutte = all, all of them (feminine plural)
• tutto = everything / all of it (singular)

So:

• Tutti arrivano = Everyone arrives / They all arrive
• Tutte arrivano = They all arrive (all women/girls)
• Tutto arriva would not fit this meaning; it would mean something like everything arrives, which is different.

In Italian, a mixed group normally uses the masculine plural, so tutti is the default form unless the group is entirely feminine.

Because the subject is tutti, which refers to all the people / everyone, so the verb must be third person plural.

The infinitive is arrivare = to arrive.

Present tense:

• arrivo = I arrive
• arrivi = you arrive
• arriva = he/she arrives
• arriviamo = we arrive
• arrivate = you all arrive
• arrivano = they arrive

So:

• Marco arriva = Marco arrives
• Tutti arrivano = Everyone arrives / They all arrive

Even though English often uses everyone with a singular verb, Italian uses tutti here as a plural idea, so arrivano is correct.

Here, tutti is being used as a pronoun.

It stands on its own and means everyone or all of them.

Compare:

• Tutti arrivano. = Everyone arrives.
  
  • Here tutti replaces a noun, so it is a pronoun.
• Tutti gli studenti arrivano. = All the students arrive.
  
  • Here tutti describes gli studenti, so it is an adjective/determiner.

So in your sentence, there is no noun after tutti, which is why it functions like everyone.

Tranne means except.

It introduces the person or thing that is excluded from the statement.

So:

• Tutti arrivano tranne Marco. = Everyone arrives except Marco.

It is commonly followed by:

• a noun: tranne Marco
• a stressed pronoun: tranne me, tranne lui, tranne noi
• sometimes a phrase: tranne oggi = except today

It is a very common and straightforward word for except in Italian.

They are very close in meaning. Both can mean except.

Examples:

• Tutti arrivano tranne Marco.
• Tutti arrivano eccetto Marco.

Both are correct and natural.

That said:

• tranne is extremely common in everyday speech
• eccetto can sound a little more formal in some contexts

For a learner, tranne is an excellent word to know and use.

Because in standard Italian, personal names usually do not take an article.

So you say:

• Marco arriva
• tranne Marco
• vedo Marco

not normally:

• il Marco

In some regional varieties of Italian, especially in parts of northern Italy, people may use an article before names in casual speech, but in standard Italian you generally do not use one.

So tranne Marco is exactly what you would expect.

Yes. That is also correct.

These versions are all possible:

• Tutti arrivano tranne Marco.
• Tutti tranne Marco arrivano.
• Arrivano tutti, tranne Marco.

The meaning stays basically the same, but the focus shifts slightly:

• Tutti arrivano tranne Marco = neutral, very natural
• Tutti tranne Marco arrivano = puts except Marco closer to tutti, so the exclusion is highlighted earlier
• Arrivano tutti, tranne Marco = can sound a little more emphatic or spoken, especially with a pause

So the original sentence is perfectly natural, but word order in Italian can move around for emphasis.

Because Italian usually does not need subject pronouns when the verb ending already shows the subject.

In arrivano, the ending -ano tells you it is they.

Also, the sentence already has tutti as its subject, so adding loro would be unnecessary.

Italian prefers:

• Tutti arrivano.

rather than:

• Loro arrivano tutti.

unless there is special emphasis or contrast.

This is very different from English, where a subject pronoun is usually required.

Yes. When tranne is followed by a pronoun, Italian uses the stressed pronoun forms, not the weak subject pronouns.

Examples:

• tranne me = except me
• tranne te = except you
• tranne lui = except him
• tranne lei = except her
• tranne noi = except us
• tranne voi = except you all
• tranne loro = except them

So if you changed the sentence, you could say:

• Tutti arrivano tranne lui. = Everyone arrives except him.

Usually, no comma is necessary in a short sentence like this:

• Tutti arrivano tranne Marco.

That is the most neutral written form.

A comma is possible if you want to mark a pause or make the exception stand out more:

• Tutti arrivano, tranne Marco.

So:

• without comma = normal and smooth
• with comma = slightly more marked or dramatic pause

Both can be correct, but the version without the comma is often the default.

Yes, the double consonant matters.

Tranne has a double n, and in Italian double consonants are pronounced more strongly/longer than single ones.

So tranne is not pronounced like a soft, quick trane. The nn should be clearly held a little:

• tran-ne

Also:

• arrivano has the stress on ri: ar-RI-va-no
• tranne has the stress on the first syllable: TRAN-ne

Paying attention to double consonants is important in Italian, because they can distinguish words and make your pronunciation sound much more natural.