Quando a curva é fechada, o motorista deve diminuir a velocidade.

Because fechada agrees with curva, and curva is a feminine noun in Portuguese.

• a curva
• curva fechada

If the noun were masculine, the adjective would normally be masculine too.

This kind of agreement is very common in Portuguese:

• o carro vermelho
• a casa vermelha

So here, fechada matches curva.

Literally, yes, fechado / fechada often means closed. But in this sentence it has a different, very common extended meaning.

With roads and turns, curva fechada means a sharp or tight curve, not a physically closed one.

So this is a good example of how a familiar word can have a more specific meaning depending on context.

Because ser is being used to describe a characteristic of the curve, not a temporary condition.

• é fechada = it is a sharp/tight curve
• está fechada would suggest something temporary, and with fechada it could even sound like the road or route is blocked/closed

In Portuguese, ser is generally used for inherent description or classification, while estar is used for states or temporary situations.

Because Portuguese often uses the present tense for general rules, repeated situations, and instructions.

This sentence is basically stating a general traffic rule:

• whenever the curve is sharp, the driver should slow down

So the present tense sounds natural.

In English, we do something similar:

• When the road is wet, drivers should be careful
• If the light is red, you must stop

Because the sentence begins with a subordinate clause introduced by quando.

Structure:

• Quando a curva é fechada, = subordinate clause
• o motorista deve diminuir a velocidade. = main clause

In Portuguese, it is standard to put a comma after this kind of introductory clause.

Portuguese uses definite articles much more often than English does.

Here, a curva and o motorista are being used in a generic sense:

• a curva = a curve / the curve, in the sense of any curve of that type
• o motorista = the driver, meaning drivers in general

This is very natural in Portuguese. English often drops the article in general statements, but Portuguese often keeps it.

No. Motorista can refer to either a male or female driver.

It is a common-gender noun:

• o motorista = the male driver
• a motorista = the female driver

So the noun itself stays the same, and the article shows the gender.

Because after dever meaning should / must / ought to, Portuguese normally uses the infinitive.

So:

• deve diminuir
• literally: should decrease / should reduce

This is a standard verb pattern:

• deve parar
• deve esperar
• deve obedecer

A conjugated verb is followed by an infinitive.

It can suggest either one depending on context, but here it usually carries the sense of a rule or obligation.

In a traffic-safety sentence, deve is often best understood as:

• should
• or must, if the tone is more regulatory

So it expresses duty, recommendation, or obligation.

Because Portuguese often states the object explicitly, especially in formal or instructional language.

• diminuir a velocidade = reduce the speed / slow down

Just diminuir can work in some contexts, but diminuir a velocidade is clearer and more complete, especially in driving language.

Again, Portuguese often uses the definite article where English might not.

Here a velocidade refers to the speed the driver is traveling at. Using the article sounds natural and complete in Portuguese.

Compare:

• reduzir a velocidade
• aumentar a velocidade
• controlar a velocidade

These are very common set expressions.

Yes. Both are correct.

• diminuir a velocidade = very natural, common, everyday
• reduzir a velocidade = also very common, slightly more formal or technical in some contexts

In traffic instructions, either one sounds normal.

The main clause follows a very normal Portuguese pattern:

• o motorista = subject
• deve diminuir = verb phrase
• a velocidade = object

So:

• o motorista deve diminuir a velocidade

The sentence starts with Quando a curva é fechada, which is an introductory time clause. After that, the main clause keeps regular word order.