Quando eu entrar na sala de aula, vou ficar em silêncio.

Because after quando referring to a future event, Portuguese normally uses the future subjunctive.

• Quando eu entrar... = When I enter / When I go in... in the future
• Quando eu entro... usually means When I enter... as a habitual or regular action, or it can sound like present-time narration

So in your sentence, the speaker is talking about something that will happen later, which is why entrar is correct.

It is the future subjunctive.

This is a very common tense in Portuguese after words like:

• quando = when
• se = if
• assim que = as soon as
• logo que = once / as soon as

Examples:

• Quando eu chegar, te ligo. = When I arrive, I’ll call you.
• Se ele vier, avise-me. = If he comes, let me know.

A useful thing to notice: for many regular verbs, the future subjunctive looks the same as the personal infinitive. So entrar here may look like the infinitive, but in this sentence it is functioning as the future subjunctive.

Yes, Portuguese often allows the subject pronoun to be omitted, so Quando entrar na sala de aula, vou ficar em silêncio is possible if the subject is already clear from context.

However, eu may be included for a few reasons:

• to make the subject extra clear
• to add emphasis
• to contrast with someone else

For example:

• Quando eu entrar, vou ficar em silêncio.
• Quando você entrar, vai fazer barulho.

So eu is not required, but it is perfectly natural.

Na is a contraction of:

• em
  • a = na

So:

• em a sala → na sala

This is extremely common in Portuguese.

Other similar contractions:

• em + o = no
• de + a = da
• de + o = do
• a + a = à

So na sala de aula literally comes from in the classroom.

Sala de aula is the standard expression for classroom.

Literally:

• sala = room
• de aula = of class / for lessons

Together, sala de aula means classroom.

A learner might expect a single-word equivalent, but Portuguese very often uses this full expression. In Brazil, sala de aula is the most common neutral term.

Both are possible, but vou ficar is much more common in everyday Brazilian Portuguese.

• vou ficar em silêncio = I’m going to stay/remain silent
• ficarei em silêncio = I will remain silent

The simple future (ficarei) is correct, but it often sounds:

• more formal
• more written
• less conversational

In spoken Brazilian Portuguese, ir + infinitive is extremely common for future meaning.

Ficar em silêncio means to remain/stay silent or to keep quiet.

Here, ficar has the idea of staying in a state.

So:

• ficar em silêncio = stay silent
• ficar calado = stay quiet / keep quiet

This does not necessarily mean becoming silent at that exact moment; it often suggests maintaining that state once the person enters.

Compare:

• vou ficar em silêncio = I’m going to remain silent
• vou estar em silêncio = I’m going to be silent

Both can work, but ficar em silêncio is very natural and common.

Yes.

• em silêncio describes how someone is acting
• silencioso is an adjective meaning silent or quiet

So:

• vou ficar em silêncio = I’m going to remain silent
• vou ficar silencioso sounds less natural for a person in this context

For people, Portuguese usually prefers expressions like:

• ficar em silêncio
• ficar calado

Silencioso is more often used for places, environments, or objects:

• um quarto silencioso = a quiet room
• uma rua silenciosa = a silent/quiet street

You could, but na sala de aula is more common and more neutral in Brazilian Portuguese.

• sala de aula = classroom
• classe can mean class or group of students, and sometimes it can mean the classroom too, depending on context

A Brazilian learner would hear sala de aula much more often for the physical room.

So for the sentence you gave, na sala de aula is the safest and most natural choice.

The comma separates the time clause from the main clause.

• Quando eu entrar na sala de aula, = when I enter the classroom
• vou ficar em silêncio. = I’m going to remain silent

This is similar to English, where a comma is often used after an introductory clause:

• When I enter the classroom, I’m going to stay silent.

In Portuguese, this comma is very normal and helps readability.

Yes. You can also say:

Vou ficar em silêncio quando eu entrar na sala de aula.

This is also correct and natural.

The difference is mostly about emphasis:

• Quando eu entrar na sala de aula, vou ficar em silêncio.
  Emphasizes the time/situation
• Vou ficar em silêncio quando eu entrar na sala de aula.
  Emphasizes the action first

Both are good Portuguese.

Because in Portuguese, after quando introducing a future event, you normally do not use ir + infinitive inside that clause. You use the future subjunctive instead.

So:

• Quando eu entrar... = correct
• Quando eu vou entrar... = usually not correct for this meaning

This is one of the big differences from English. English often uses present forms after when:

• When I enter...
• not usually When I will enter...

Portuguese behaves similarly, but with its own special form: the future subjunctive.

Yes. That would also be natural.

• quando = when
• assim que = as soon as / once

So:

• Quando eu entrar na sala de aula, vou ficar em silêncio.
• Assim que eu entrar na sala de aula, vou ficar em silêncio.

The second version feels a bit more immediate, as if the speaker will become silent right at that moment.

Both expressions are followed by the future subjunctive, so you still use entrar.