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

When I go into the classroom, I will stay quiet.

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

• eu: I
• ir: to go (future auxiliary)
• quando: when
• ficar: to stay

Questions & Answers about Quando eu entrar na sala de aula, vou ficar em silêncio.

Why is it quando eu entrar and not quando eu entro?

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.

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What tense/mood is entrar here?

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.

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Why does the sentence include eu? Could it just be Quando entrar na sala de aula...?