Quando chove, o motorista deve diminuir a velocidade.

Both are possible, but they mean slightly different things.

• Quando chove = when it rains
  This presents rain as a general or recurring situation.
• Se chover = if it rains
  This sounds more conditional, referring to a possible future situation.

So in a general safety rule, quando chove is very natural:

• Quando chove, o motorista deve diminuir a velocidade.

If you were talking about a specific future possibility, you might say:

• Se chover, o motorista deverá diminuir a velocidade.

Chover is an impersonal verb when it refers to weather, just like to rain in English.

That means it normally appears only in the 3rd person singular:

• chove = it rains / it is raining

But unlike English, Portuguese usually does not use a dummy subject like it. So:

• English: It rains
• Portuguese: Chove

There is no real subject in the sentence.

Because chove here expresses a general condition, not an action in progress at one exact moment.

• Quando chove = when it rains / whenever it rains
• Quando está chovendo = when it is raining

Both can be correct, but they are used a little differently:

• Quando chove, o motorista deve diminuir a velocidade.
  General rule.
• Agora está chovendo.
  It is raining now.

In this sentence, the simple present chove is the most natural choice.

Not necessarily. In Portuguese, the definite article is often used to talk about a person or thing in a general sense.

So o motorista here can mean:

• the driver in a general, generic sense
• essentially a driver / any driver

This is very common in Portuguese.

Compare:

• O motorista deve diminuir a velocidade.
  = The driver / A driver should reduce speed.
• Motorista deve diminuir a velocidade.
  This is possible in some contexts, especially in headlines or signs, but the version with o sounds more like standard full-sentence Portuguese.

Deve can express obligation, recommendation, or expected behavior, depending on context.

In this sentence, it usually means something like:

• should
• must
• is supposed to

Because this is a traffic-safety statement, English could translate it in different ways depending on tone:

• the driver should reduce speed
• the driver must reduce speed

Grammatically, deve is the present tense of dever.

In Portuguese, it is very common to use the definite article with nouns where English may omit it.

So:

• diminuir a velocidade = reduce the speed / slow down

This sounds natural and complete in Portuguese.

Without the article:

• diminuir velocidade would sound unnatural in normal standard speech.

All of these are possible, but they are a bit different in tone and frequency.

• diminuir a velocidade = very common, clear, natural
• reduzir a velocidade = also very common, slightly more formal
• desacelerar = to decelerate; correct, but less everyday in many contexts

So diminuir a velocidade is a very natural choice for learner-friendly, everyday Portuguese.

Because quando chove is an introductory subordinate clause.

Portuguese normally uses a comma when a dependent clause comes before the main clause:

• Quando chove, o motorista deve diminuir a velocidade.

If you reverse the order, the comma is often not used:

• O motorista deve diminuir a velocidade quando chove.

So the comma here is standard and expected.

Grammatically, Portuguese often allows omitted subjects, but in this sentence that would be less clear.

• Quando chove, o motorista deve diminuir a velocidade.
  Clear: the driver is the one who should slow down.
• Quando chove, deve diminuir a velocidade.
  Possible in some contexts, but it can sound incomplete or vague unless the subject is obvious from context.

A more impersonal way to say it would be:

• Quando chove, deve-se diminuir a velocidade.

That means something like:

• When it rains, one should reduce speed
• When it rains, speed should be reduced

The infinitive is diminuir.

It appears after deve because Portuguese, like English, often uses a structure with a conjugated modal-like verb plus an infinitive:

• deve diminuir = should reduce / must reduce

Here:

• deve is conjugated for o motorista
• diminuir stays in the infinitive

This is similar to English:

• The driver should reduce speed

No. The correct spelling is velocidade.

A few useful notes:

• velocidade = speed
• It is a feminine noun, which is why it takes a:
  • a velocidade

So:

• a velocidade = the speed

The original sentence is already natural and correct. But in everyday Brazilian Portuguese, people might also say:

• Quando chove, o motorista tem que diminuir a velocidade.
• Quando chove, é preciso diminuir a velocidade.
• Quando está chovendo, o motorista precisa reduzir a velocidade.

These are all natural, but the original version is especially good for standard, neutral Portuguese.

It sounds like a general rule.

That effect comes from:

• quando chove = whenever/when it rains
• present tense verbs: chove and deve

So the sentence is not mainly about one specific rainy moment. It gives a rule or recommendation that applies in general.