Dopo la lezione di nuoto, Martina torna nello spogliatoio e si cambia con calma.

This opening phrase gives the time context: it means after the swimming lesson.

• dopo = after
• la lezione = the lesson
• di nuoto = of swimming / swimming

Italian often begins a sentence with a time phrase like this, just as English can: After the swimming lesson, ...

The comma is natural here because this introductory phrase is fairly long.

Di nuoto is the normal way to specify what kind of lesson it is.

So:

• lezione di nuoto = swimming lesson
• literally, lesson of swimming

In Italian, di is very often used to describe the type or subject of something:

• corso di italiano = Italian course
• scarpe da tennis = tennis shoes

Notice that da is used for purpose or function in many cases, but here we are naming the subject of the lesson, so di is the right choice.

Torna is the third-person singular present tense of tornare.

Here it means she returns / she goes back.

So:

• io torno = I return
• tu torni = you return
• lei torna = she returns

Italian often uses the present tense to describe actions in a straightforward way, especially in examples, summaries, routines, or narrative-style sentences. So even if English might sometimes say Martina goes back, Italian uses the present naturally here.

In this sentence, tornare means to return or to go back.

So Martina torna nello spogliatoio means that Martina goes back to the changing room.

Depending on context, tornare can also mean:

• to come back
• to return
• to go back

The exact English translation depends on perspective, but the Italian verb stays the same.

Because spogliatoio begins with sp-, and nouns beginning with s + consonant usually take lo as the article, not il.

So:

• lo spogliatoio = the changing room
• in + lo = nello

This is a normal contraction in Italian.

A few similar examples:

• nello stadio
• nello zaino
• nello specchio

If the noun took il, then in + il would become nel. But spogliatoio takes lo, so we get nello.

Spogliatoio means changing room or locker room.

It comes from the idea of getting undressed or changed. In this sentence, it is the place Martina goes after the swimming lesson to change her clothes.

Because the verb here is used reflexively: cambiarsi.

That means the action comes back to the subject:

• cambia = changes
• si cambia = changes herself / gets changed / changes clothes

In everyday Italian, cambiarsi is very common when talking about changing clothes.

So:

• Martina si cambia = Martina gets changed / changes her clothes

Without si, cambia would usually need a direct object or would sound less natural in this context.

Grammatically, yes, it is reflexive, so literally it looks like she changes herself. But in natural English, that is not how we usually say it.

A better translation is:

• she gets changed
• she changes her clothes

So the reflexive form in Italian often corresponds to a more natural expression in English rather than a word-for-word reflexive translation.

Si is the reflexive pronoun for he/she/it and also for the formal you.

In this sentence it refers back to Martina.

With cambiarsi, the reflexive pronouns are:

• mi cambio = I get changed
• ti cambi = you get changed
• si cambia = he/she gets changed
• ci cambiamo = we get changed
• vi cambiate = you all get changed
• si cambiano = they get changed

So si is necessary because the verb is being used reflexively.

Con calma means calmly, slowly, or without rushing.

Literally it is with calm.

It is a very common Italian expression. In this sentence, it tells us how Martina changes:

• si cambia con calma = she gets changed calmly / she changes without hurrying

Italian often uses a prepositional phrase like con calma where English might prefer an adverb such as calmly.

This is a very normal Italian word order:

• subject: Martina
• verb: torna
• place: nello spogliatoio
• second verb phrase: e si cambia con calma

It is basically the same kind of structure as in English: Martina goes back to the changing room and gets changed calmly.

Italian word order can sometimes be flexible, but this version is the most neutral and natural.

Because Martina is a proper name, and in standard Italian proper names usually do not take an article.

So you simply say:

• Martina torna...

not normally

• La Martina torna...

In some regional varieties of Italian, especially in parts of northern Italy, people may use an article before a person’s name in speech, but in standard Italian it is usually omitted.

Yes, you may sometimes see it without a comma, especially in shorter or less formal writing.

However, the comma is very natural here because Dopo la lezione di nuoto is an introductory phrase. It helps separate the time expression from the main clause and makes the sentence easier to read.

So:

• Dopo la lezione di nuoto, Martina torna... = very natural
• Dopo la lezione di nuoto Martina torna... = also possible

The meaning does not change.