Quando la preside entra in aula, nessuno si mette a parlare.

Preside is a noun for a school principal/head teacher. In modern Italian, it can be used for either a man or a woman, and the article usually shows the gender:

• il preside = a male principal
• la preside = a female principal

So la preside tells you the principal is a woman.

Italian often uses the present tense for actions that happen regularly or generally, not just for what is happening right now.

So Quando la preside entra in aula, nessuno si mette a parlare is naturally understood as something like a habitual situation: whenever the principal comes into the classroom, nobody starts talking.

This is very normal in both Italian and English:

• When the teacher arrives, everyone goes quiet.
• Quando l’insegnante arriva, tutti stanno zitti.

Quando is the normal word for when in time clauses.

Here it introduces a subordinate clause:

• Quando la preside entra in aula = when the principal enters the classroom

It is the standard choice for this kind of sentence.

In Italian, after quando referring to a future or repeated event, the present tense is often used where English might sometimes use a future idea.

Italian prefers:

• Quando arriva, ti chiamo.
• literally: When he arrives, I call you
• natural meaning: When he arrives, I’ll call you

In your sentence, the present tense works because the sentence describes a general repeated situation.

Aula means classroom, lecture room, or sometimes schoolroom, depending on context.

The expression entrare in aula is very common and often appears without an article. Italian often omits the article in certain fixed or semi-fixed expressions of place, especially after verbs of movement.

So:

• entrare in aula = to go into the classroom
• essere in aula = to be in the classroom

You may also hear forms with an article in more specific contexts, but in aula is very natural here.

Nessuno is the normal, everyday way to say nobody / no one.

It is much more natural than a fuller phrase like nessuna persona, which sounds more formal or emphatic.

So:

• nessuno = nobody / no one

It is the simplest and most idiomatic choice here.

Because nessuno is grammatically singular in Italian, even though it refers to zero people.

So you say:

• Nessuno parla. = Nobody speaks.
• Nessuno entra. = Nobody enters.

That is why you get:

• nessuno si mette a parlare not
• nessuno si mettono a parlare

Because nessuno comes before the verb.

In Italian, negative words like nessuno, niente, mai usually do not need non when they come before the verb:

• Nessuno parla. = Nobody speaks.
• Niente è successo. = Nothing happened.

But if they come after the verb, then you normally use non:

• Non parla nessuno. = Nobody is speaking.
• Non ho visto nessuno. = I didn’t see anyone.

So nessuno si mette a parlare is perfectly correct without non.

Mettersi a + infinitive is a very common Italian structure meaning to start / begin to do something, often with a sense of suddenly or actively setting oneself to it.

So:

• mettersi a parlare = to start talking
• mettersi a ridere = to start laughing
• mettersi a studiare = to start studying

In your sentence, nessuno si mette a parlare means that nobody starts talking.

The verb here is mettersi, not just mettere.

• mettere = to put
• mettersi a + infinitive = to start doing something

So the si is part of the pronominal/reflexive verb mettersi. You should learn mettersi a as a whole pattern.

Examples:

• Mi metto a leggere. = I start reading.
• Si mette a correre. = He/She starts running.
• Ci mettiamo a lavorare. = We start working.

Because the structure is fixed:

• mettersi a + infinitive

So you must say:

• si mette a parlare
• si mette a ridere
• si mette a urlare

Not:

• si mette parlare
• si mette di parlare

Different verbs take different prepositions before an infinitive, and mettersi takes a in this meaning.

They are close, but not always identical in tone.

• cominciare a parlare = to begin to speak/talk
• mettersi a parlare = to start talking

Mettersi a can feel a little more dynamic, sometimes suggesting that someone actively launches into the action. In many contexts, though, the difference is small.

So in this sentence:

• nessuno si mette a parlare suggests nobody starts talking once the principal comes in.

Parlare is the general verb to speak / to talk.

Depending on context, it can simply mean speaking, but here it often suggests starting to talk out of turn or starting to say something.

If the speaker wanted to stress chatting or chattering, they might choose:

• chiacchierare

But parlare is broader and more neutral.

The comma separates the quando clause from the main clause:

• Quando la preside entra in aula, = subordinate time clause
• nessuno si mette a parlare. = main clause

This is very common when the time clause comes first.

You could also reverse the order:

• Nessuno si mette a parlare quando la preside entra in aula.

That is also grammatical, though the original version is often clearer and more natural in writing.

You can understand the pieces literally, but the most natural English translation may sound a bit different.

Piece by piece:

• Quando = when
• la preside = the principal
• entra = enters
• in aula = into the classroom
• nessuno = nobody
• si mette a parlare = starts talking

A very literal version would be:

• When the principal enters the classroom, nobody starts talking.

That is understandable, but depending on context, natural English might also be:

• When the principal comes into the classroom, nobody starts talking.
• When the principal walks in, no one starts talking.

Not always. Aula can mean several kinds of formal learning or institutional rooms, such as:

• a classroom
• a lecture room
• a schoolroom
• in other contexts, even a courtroom or hall

In this sentence, because of la preside, the most likely meaning is a school classroom.

Yes, but it would not mean exactly the same thing.

• nessuno parla = nobody speaks / nobody is talking
• nessuno si mette a parlare = nobody starts talking

The original sentence focuses on the beginning of the action. It suggests that once the principal enters, no one even starts to speak.

So si mette a parlare is a little more specific than just parla.