Learn Hardcore Portuguese (Portugal): O aquecedor será desligado assim que a temperatura subir. - The heater will be turned off as soon as the temperature rises.

será desligado is the simple future passive voice, formed by the future of ser (será) + the past participle desligado. It means “will be turned off,” focusing on the action happening to the heater rather than who performs it.

Why is desligado used here? Does it mean the heater is already off?

No. desligado is a past participle used with ser to build the passive. It doesn’t imply the action is complete now; combined with será, it signals that the action will happen in the future.

Why is the verb after assim que in the infinitive (subir) instead of a conjugated form like suba or sobe?

Here subir is a personal infinitive because its subject (a temperatura) is explicitly stated before the verb. Portuguese lets you mark the subject of an infinitive by placing a noun or pronoun ahead of it. You could also use the present subjunctive—assim que a temperatura suba—which is equally correct and often preferred for future events in speech.

Can I replace assim que with quando? Would that change the meaning or grammar?

Yes. quando, assim que, logo que, and tão logo que all mean “as soon as” in future contexts. After any of these conjunctions, you still use either the personal infinitive or present subjunctive for an event that has not yet occurred (e.g. quando a temperatura subir/suba). Assim que can sound a bit more formal or literary than quando.

What’s the difference between será desligado and vai ser desligado?

Both express the future passive (“will be turned off.”)

será desligado is the simple future—more common in formal/written Portuguese.
vai ser desligado is the periphrastic or near future (ir + infinitive)—used frequently in everyday speech.
In practice, they are interchangeable.

Why does Portuguese use the definite article o before aquecedor? In English we’d say “heater” without “the.”

Portuguese routinely uses definite articles before nouns—even devices and appliances. So o aquecedor can be understood generically (“the heater,” meaning the device in question). Omitting the article in European Portuguese would sound unusual here.

Could the sentence be written with the subordinate clause first, like Assim que a temperatura subir, o aquecedor será desligado?

Yes. The meaning and grammar remain the same. Placing Assim que a temperatura subir at the beginning simply shifts the emphasis slightly—this word order is common in both formal writing and technical descriptions.

Why is a temperatura placed before subir? Would subir a temperatura work?

Putting a temperatura before subir marks it as the subject of the personal infinitive (“the temperature rises”). Saying subir a temperatura makes subir a transitive verb (“to raise something”) and a temperatura its object (“to raise the temperature”), which changes the meaning. The original order is needed to show that the temperature itself goes up.