Tutti sono in casa, salvo il cane.

Tutti means everyone or literally all. It is plural because it refers to a group of people.

In Italian, tutti is the masculine plural form. It is used:

• for a group of males,
• for a mixed group of males and females,
• sometimes in a general sense like everyone.

If the group were all female, you would say tutte.

So:

• Tutti sono in casa = Everyone is at home / in the house
• Tutte sono in casa = All of them are at home, if they are all female

Because tutti is grammatically plural, the verb must also be plural.

The verb is essere = to be:

• è = is (singular)
• sono = are (plural)

So:

• Tutti sono = Everyone is / All are
• not Tutti è, because that would be a mismatch in number

Even though English often uses singular everyone is, Italian uses the plural structure here with tutti.

Both in casa and a casa are possible, but they are not always felt in exactly the same way.

In this sentence, in casa suggests in the house / inside the home. It emphasizes being physically inside.

A casa often means at home in a broader, more idiomatic sense.

So:

• Tutti sono in casa = everyone is inside the house
• Tutti sono a casa = everyone is at home

In many situations, the difference is small, but in casa sounds a bit more literal and spatial.

Because in casa is a very common fixed expression in Italian.

Italian often omits the article with certain places or expressions, especially when they are used in a general or idiomatic way. Casa is one of the most common examples.

Compare:

• sono in casa = they are at home / in the house
• sono nella casa = they are in the house

Nella casa is more specific and means in the house with emphasis on a particular house.
In casa is more natural for the general idea of being at home or indoors.

Here salvo means except for, apart from, or with the exception of.

So:

• Tutti sono in casa, salvo il cane = Everyone is in the house, except the dog

In this sentence, salvo is functioning like a preposition or exception marker.

It is a bit more formal or literary than everyday tranne or eccetto, but it is perfectly correct.

It is correct and understandable, but in everyday speech many speakers would more naturally say:

• tranne il cane
• eccetto il cane

So these are all possible:

• Tutti sono in casa, salvo il cane
• Tutti sono in casa, tranne il cane
• Tutti sono in casa, eccetto il cane

Salvo can sound slightly more formal, written, or elegant.

Italian often uses the definite article with nouns in places where English may or may not use it.

Here il cane means the dog. The sentence is referring to a specific dog already known in the context.

So:

• salvo il cane = except the dog

Without the article, salvo cane would not sound natural here.

In this sentence, salvo is being used as a preposition-like word meaning except for, and in that use it is usually left as salvo.

However, salvo can also be an adjective meaning safe or saved, and then it does change:

• salvo = masculine singular
• salva = feminine singular
• salvi = masculine plural
• salve = feminine plural

But that is a different use.

In Tutti sono in casa, salvo il cane, you should understand salvo simply as except for.

The comma is very natural here because salvo il cane is an added exception phrase.

So:

• Tutti sono in casa, salvo il cane. is standard and clear.

You may sometimes see similar expressions without a comma, especially in less careful writing, but with this structure the comma is a good choice.

Yes. Italian is fairly flexible with word order.

For example, you could also say:

• Salvo il cane, tutti sono in casa.

This means the same thing: Except for the dog, everyone is in the house.

The original version puts the main statement first and the exception afterward.
The alternative puts more emphasis on the exception right away.

Not always. Tutti can refer to people or things, depending on context.

For example:

• Tutti sono pronti = everyone is ready
• Tutti i libri sono sul tavolo = all the books are on the table

In your sentence, because of sono in casa, the natural interpretation is a group of people, with the dog as the one exception.

Then tutti would become tutte:

• Tutte sono in casa, salvo il cane.

Everything else stays the same.

That is because tutte is the feminine plural form. The verb sono does not change, because it is already the plural form for both masculine and feminine subjects.