En cuanto la profesora entra en el aula, dejamos de hablar.

En cuanto means as soon as or the moment in this kind of sentence.

So:

• En cuanto la profesora entra... = As soon as the teacher enters...

It is a very common time expression in Spanish. Other similar options are:

• tan pronto como
• apenas
• sometimes cuando, depending on context

But en cuanto is a very natural choice here.

Because this sentence is talking about a habitual action or something that regularly happens, not a future event.

• En cuanto la profesora entra en el aula, dejamos de hablar.
  = Whenever / as soon as the teacher comes into the classroom, we stop talking.

Here, Spanish uses the present indicative: entra.

But if you were talking about the future, Spanish would normally use the present subjunctive after en cuanto:

• En cuanto la profesora entre en el aula, dejaremos de hablar.
  = As soon as the teacher enters the classroom, we will stop talking.

So the contrast is:

• habit / repeated situation → entra
• future event → entre

For the same reason: the sentence describes something that normally happens.

• la profesora entra = the teacher enters
• dejamos de hablar = we stop talking

Spanish often uses the present tense to describe regular or repeated actions, just like English can do:

• When she arrives, we start.
• As soon as the teacher enters, we stop talking.

So dejamos does not mean only we leave here. In this structure, it means we stop.

On its own, dejar often means to leave, to let, or to leave behind. But in the structure:

• dejar de + infinitive

it means to stop doing something.

So:

• dejar de hablar = to stop talking
• dejar de fumar = to stop smoking
• dejar de estudiar = to stop studying

In your sentence:

• dejamos de hablar = we stop talking

This is a very important fixed pattern to learn.

Because entrar en is the normal pattern here in Spanish.

• entrar en el aula = to enter the classroom / go into the classroom

Spanish usually says entrar en a place when focusing on going inside it.

You may also hear entrar al aula in some varieties of Spanish, but in Spain entrar en el aula is especially standard and natural.

So for a learner of Spanish from Spain, entra en el aula is an excellent model.

Yes, aula is feminine.

That is why we say:

• la profesora = feminine
• el aula = still feminine, even though it uses el

This happens because aula is a feminine noun that begins with a stressed a- sound. In the singular, Spanish often uses el instead of la before such nouns to make pronunciation easier.

Examples:

• el aula
• el agua
• el águila

But they are still feminine, so adjectives agree in the feminine:

• el aula pequeña
• el agua fría

And in the plural, normal feminine articles return:

• las aulas
• las aguas

So el aula is correct, but the noun is still feminine.

Because Spanish often drops subject pronouns when the verb ending already shows who the subject is.

• dejamos already tells you it means we
• so nosotros is not necessary

That is why:

• dejamos de hablar

sounds completely natural.

If you add nosotros, it usually gives extra emphasis or contrast:

• Nosotros dejamos de hablar, pero ellos no.
  = We stop talking, but they don’t.

So in your sentence, leaving out nosotros is normal.

Yes, you could, and it would be grammatical. But the nuance is slightly different.

• En cuanto la profesora entra... = As soon as the teacher enters...
• Cuando la profesora entra... = When the teacher enters...

En cuanto stresses the idea of immediacy more strongly. It suggests that the second action happens right away.

So if you want the sense of the moment she comes in, we stop, en cuanto is better.

Because the sentence begins with a subordinate time clause:

• En cuanto la profesora entra en el aula, ...

After that introductory clause, a comma is commonly used before the main clause:

• ..., dejamos de hablar.

This is similar to English:

• As soon as the teacher enters the classroom, we stop talking.

If the main clause came first, the comma would often disappear:

• Dejamos de hablar en cuanto la profesora entra en el aula.

In this sentence, hablar is best understood as to talk.

• dejamos de hablar = we stop talking

In many contexts, hablar can mean either to speak or to talk, depending on the situation.

Examples:

• Hablo español. = I speak Spanish.
• Estamos hablando. = We are talking.

Here, because the class stops making noise when the teacher comes in, talking is the most natural translation.

Yes.

• profesora = female teacher
• profesor = male teacher

Spanish nouns often show gender this way. Since the sentence says la profesora, we know the teacher is female.

In English, the teacher does not usually specify gender, but Spanish does here.

Yes. You can also say:

• Dejamos de hablar en cuanto la profesora entra en el aula.

This means the same thing.

The difference is mostly one of focus and sentence flow:

• En cuanto la profesora entra en el aula, dejamos de hablar.
  This puts the time condition first.
• Dejamos de hablar en cuanto la profesora entra en el aula.
  This starts with the main action.

Both are correct and natural.

Most naturally, it describes a repeated or habitual situation.

It sounds like:

• every time the teacher enters the classroom, we stop talking

That is why both verbs are in the present indicative:

• entra
• dejamos

If you wanted to describe a single event in the past, you would use past tenses instead, for example:

• En cuanto la profesora entró en el aula, dejamos de hablar.
  = As soon as the teacher entered the classroom, we stopped talking.

So the original sentence most likely expresses a general pattern.