Marta ha comprato un altro rotolo di carta e una bustina di caramelle perché i bambini possano fare merenda in treno.

Ha comprato is the passato prossimo, a very common Italian past tense used for completed actions.

It is made with:

• the present tense of avere or essere
• plus the past participle

Here:

• ha = she has
• comprato = bought

So Marta ha comprato means Marta bought / has bought.

Italian often uses this structure where English might simply use bought.

Because altro is not being elided here.

• un altro = another
• un is the normal masculine singular indefinite article
• altro is an adjective meaning other or another

So:

• un altro rotolo = another roll

By contrast, you may see un'altra with a feminine noun:

• un'altra bustina = another little packet

That apostrophe appears because una altra becomes un'altra.

In Italian, altro usually comes after the indefinite article when it means another:

• un altro libro = another book
• una altra idea / un'altra idea = another idea

So un altro rotolo is the normal word order.

Rotolo means roll.

So un rotolo di carta literally means a roll of paper.

This is a very common Italian pattern:

• rotolo di carta
• bicchiere d'acqua
• tazza di tè
• bottiglia di vino

The structure is:

• container/unit noun + di
  • material/content

Even if the exact meaning has already been given, grammatically it works like a roll of paper in English.

Bustina is a diminutive form of busta.

• busta = envelope, bag, packet, pouch
• bustina = little packet, small sachet, small bag

The ending -ina often adds the idea of small.

So una bustina di caramelle suggests a small packet of sweets/candies, not a large bag.

This kind of diminutive is very common in Italian:

• casa → casina
• borsa → borsina
• busta → bustina

Sometimes it means literally smaller; sometimes it can also sound more natural or affectionate.

After nouns like bustina, scatola, bicchiere, bottiglia, etto, chilo, Italian usually uses plain di to show the contents or substance.

So:

• una bustina di caramelle = a little packet of sweets
• una scatola di biscotti = a box of biscuits
• un bicchiere d'acqua = a glass of water

Using delle would usually sound less natural here.

Perché can mean either:

• because
• so that / in order that

Here it introduces a purpose, not a cause:

• Marta bought these things so that the children can have a snack on the train.

A big clue is that it is followed by the subjunctive:

• perché i bambini possano...

When perché means in order that / so that, the subjunctive is very common.

Because this clause uses the subjunctive, not the normal indicative.

• possono = present indicative of potere
• possano = present subjunctive of potere

The sentence expresses purpose:

• perché i bambini possano fare merenda
• so that the children can have a snack

After perché meaning so that, Italian normally uses the subjunctive.

So:

• perché i bambini possono... would sound wrong here
• perché i bambini possano... is the correct form

It comes from the verb potere = to be able to / can.

Present subjunctive of potere:

• che io possa
• che tu possa
• che lui/lei possa
• che noi possiamo
• che voi possiate
• che loro possano

Since i bambini is third person plural, we use:

• possano

So:

• i bambini possano = the children may be able to / can

Italian often drops subject pronouns, but here i bambini is not a pronoun like they. It is the full noun phrase the children, and it helps make the sentence clear.

Without it, the clause would be:

• perché possano fare merenda

That is grammatically possible, but the listener would have to infer who possano refers to. Since the sentence wants to specify that the children are the ones who will have the snack, i bambini is included.

Fare merenda is a fixed, very common Italian expression meaning to have a snack.

So:

• fare colazione = have breakfast
• fare merenda = have a snack

Usually no article is used in this expression.

• i bambini possano fare merenda = the children can have a snack

You may occasionally meet other ways of expressing similar ideas, but fare merenda is the standard everyday phrasing.

With means of transport, Italian often uses in:

• in treno = by train / on the train
• in autobus
• in macchina

So fare merenda in treno means to have a snack on the train / while traveling by train.

Nel treno would usually sound more like inside the train in a more literal, physical sense, and it is less natural here.

Because perché i bambini possano fare merenda in treno is tightly connected to the main clause and expresses the purpose of Marta’s action.

Italian punctuation is often a bit lighter than English punctuation in this kind of sentence. A comma is usually not necessary before a purpose clause like this.

So the sentence flows naturally as one unit:

• Marta ha comprato ... perché i bambini possano fare merenda in treno.

Yes, that is possible in Italian, but it changes the structure.

Original:

• perché i bambini possano fare merenda
• literally: so that the children may have a snack

Alternative:

• per far fare merenda ai bambini
• literally: to make the children have a snack / so as to let the children have a snack

The original sentence is more straightforward and very natural. It uses a full clause with perché + subjunctive. The alternative uses an infinitive construction and sounds a bit more complex.

Yes, it is very natural.

The structure is:

1. subject: Marta
2. verb: ha comprato
3. objects: un altro rotolo di carta e una bustina di caramelle
4. purpose clause: perché i bambini possano fare merenda in treno

This is a normal, clear Italian sentence. A native speaker would find the word order completely natural.