Resto in casa tutta la mattina.

Resto is the first-person singular present form of restare.

• restare = to stay, to remain
• resto = I stay / I remain

So in this sentence, resto means I stay.

It follows the regular -are pattern in the present tense:

• io resto
• tu resti
• lui/lei resta
• noi restiamo
• voi restate
• loro restano

Italian often leaves out the subject pronoun when it is already clear from the verb ending.

Here, resto already tells you the subject is I, because the -o ending marks first person singular.

So:

• Resto in casa tutta la mattina.
• Io resto in casa tutta la mattina.

Both are correct, but the version without io is more natural unless you want emphasis, contrast, or clarity.

For example:

• Io resto in casa, tu esci.
  = I stay at home, you go out.

Because restare and essere do different jobs.

• essere = to be
• restare = to stay, remain

In English, I am at home all morning can sometimes suggest the same idea as I stay at home all morning, but in Italian you usually need a verb that expresses remaining/staying.

So:

• Resto in casa tutta la mattina. = I stay at home all morning.

If you said:

• Sono in casa tutta la mattina

it would mean I am at home all morning, which is possible, but it sounds more like describing where you are, not emphasizing the idea of staying there.

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

• a casa usually means at home
• in casa literally means in the house / inside the house

In many situations, a casa is the most common way to say at home.
But in casa is also very natural, especially when the idea is being indoors or inside the home.

So:

• Resto a casa tutta la mattina. = I stay at home all morning.
• Resto in casa tutta la mattina. = I stay in the house / stay at home all morning.

In this sentence, in casa gives a slight sense of staying inside.

Because casa often appears without an article in common expressions, especially when it means home rather than a specific house.

This is very common in Italian:

• andare a casa = to go home
• essere a casa = to be at home
• restare in casa = to stay at home / indoors

If you add an article, the meaning can become more specific:

• nella casa = in the house
• nella mia casa = in my house

So in casa here is a natural idiomatic expression meaning at home / indoors, not inside a particular house in a strongly literal way.

Tutta la mattina means all morning or the whole morning.

It is made of:

• tutta = all / whole
• la mattina = the morning

Literally, it is the whole morning.

This structure is very common in Italian:

• tutto il giorno = all day
• tutta la notte = all night
• tutta la settimana = all week

Because mattina is feminine singular, the adjective changes to match it:

• tutta
  • la mattina

Compare:

• tutto il pomeriggio
• tutta la serata

Because tutto has to agree with the noun it describes.

Mattina is:

• feminine
• singular

So you need the feminine singular form:

• tutta la mattina

The forms are:

• tutto = masculine singular
• tutta = feminine singular
• tutti = masculine plural
• tutte = feminine plural

Examples:

• tutto il giorno = the whole day
• tutta la mattina = the whole morning
• tutti i giorni = every day / all the days
• tutte le mattine = every morning / all the mornings

Yes, you could say:

• Resto in casa per tutta la mattina.

This is also correct. Adding per makes the duration a little more explicit: for the whole morning.

Compare:

• Resto in casa tutta la mattina.
• Resto in casa per tutta la mattina.

Both are natural. The version without per is very common and often slightly more streamlined in everyday speech.

It can do either, depending on context.

The Italian present tense often covers:

• what is happening now
• a habitual action
• a near-future plan

So Resto in casa tutta la mattina could mean:

• I’m staying at home all morning
• I stay at home all morning
• I’ll stay at home all morning

Context tells you which one is meant.

For example:

• If someone asks what you are doing today, it can mean a future plan.
• If someone asks about your usual routine, it can sound habitual.
• If you are already at home and talking about the current day, it can describe the present situation.

Yes. Italian word order is flexible, although some orders sound more neutral than others.

The most neutral version here is:

• Resto in casa tutta la mattina.

But you could also say:

• Tutta la mattina resto in casa.
• Resto tutta la mattina in casa.

These alternatives may shift emphasis slightly:

• Tutta la mattina at the beginning emphasizes the time span.
• in casa near the end can sound a bit more marked.

So the original sentence is a very natural, standard order, but it is not the only possible one.