Quando a sirene tocar, o motorista deve diminuir a velocidade.

Because in Portuguese, quando often triggers the future subjunctive when it refers to a future event.

So in this sentence, Quando a sirene tocar... means when the siren goes off / when the siren sounds in the future.

Compare:

• Quando a sirene toca, o motorista diminui a velocidade.
  = When the siren sounds, the driver slows down.
  This sounds more like a general or habitual fact.

• Quando a sirene tocar, o motorista deve diminuir a velocidade.
  = When the siren sounds, in a future situation, the driver should/must slow down.

So tocar here is not just the infinitive in meaning; it is the future subjunctive form.

This is a very common point of confusion.

For many -ar verbs, the future subjunctive looks exactly the same as the personal infinitive and the plain infinitive in some forms.

With tocar:

• infinitive: tocar
• future subjunctive, eu: tocar
• future subjunctive, você/ele/ela: tocar

So in this sentence, you recognize it from the structure, not just the form itself:

• Quando a sirene tocar...

After quando referring to the future, this is understood as future subjunctive, not an infinitive.

Here, tocar means something like:

• to sound
• to go off
• to ring

Since the subject is a sirene, tocar is being used the way English uses the siren sounds or the siren goes off.

This verb is very flexible in Portuguese. It can also mean:

• to touch
• to play an instrument
• to ring (for bells, phones, alarms, etc.)

So the meaning depends a lot on context.

Deve comes from dever.

In this sentence, deve diminuir a velocidade can mean:

• should slow down
• must slow down
• is supposed to slow down

The exact strength depends on context.

In a traffic or safety context, deve often sounds like an instruction, rule, or obligation, so it can feel close to must. But grammatically, deve is often translated as should or must, depending on tone.

A few comparisons:

• deve diminuir = should/must slow down
• tem que diminuir = has to slow down
• precisa diminuir = needs to slow down

Portuguese uses definite articles much more often than English.

So where English might say:

• When siren sounds, driver must slow down
  (which sounds unnatural in English anyway)

Portuguese naturally says:

• Quando a sirene tocar, o motorista deve diminuir a velocidade.

The articles a and o are normal here. They make the sentence sound complete and natural.

Portuguese often uses articles before nouns in general statements where English may not.

Because the sentence starts with a subordinate clause:

• Quando a sirene tocar, = When the siren sounds,

Then comes the main clause:

• o motorista deve diminuir a velocidade.

In Portuguese, when this kind of time clause comes first, it is very common to separate it with a comma.

So the comma helps show the structure:

• first, the condition/time frame
• then, the main statement

If you reverse the order, the comma may disappear:

• O motorista deve diminuir a velocidade quando a sirene tocar.

That is also correct.

Yes, both are possible.

• diminuir a velocidade = to decrease / lower / slow the speed
• reduzir a velocidade = to reduce speed

Both are natural, but diminuir a velocidade is very common in everyday Brazilian Portuguese.

You may also hear:

• reduza a velocidade on signs
• diminua a velocidade on signs
• desacelerar = to slow down

So diminuir a velocidade is a very standard and natural choice.

Again, Portuguese often prefers the article where English does not.

So:

• diminuir a velocidade literally looks like decrease the speed
• but in natural English we usually say slow down or reduce speed

In Portuguese, a velocidade is the normal expression here.

Not necessarily.

Motorista is often treated as a common-gender noun. The article shows the gender:

• o motorista = the male driver / the driver (masculine form used)
• a motorista = the female driver

In many general statements, Portuguese often uses the masculine form as a default or generic form, especially in traditional or formal language.

So here o motorista can mean:

• a male driver specifically, or
• the driver in a general sense

If you wanted to make it explicitly feminine, you would say:

• Quando a sirene tocar, a motorista deve diminuir a velocidade.

It can be understood as a general rule.

Even though the sentence uses singular nouns:

• a sirene
• o motorista

Portuguese often uses the singular with the definite article to express something general, especially in formal statements, instructions, or rules.

So it works like:

• When the siren sounds, the driver must/should slow down.

This does not necessarily mean one specific driver. It can mean any driver in that situation.

Then you would usually use the present indicative:

• Quando a sirene toca, o motorista diminui a velocidade.

This means something like:

• When the siren sounds, the driver slows down.
• Whenever the siren sounds, the driver slows down.

So the contrast is:

• quando ... tocar = future event
• quando ... toca = habitual/general event

That contrast is very important in Portuguese.

In Brazilian Portuguese, sirene is usually pronounced approximately like:

• see-REH-nee

More precisely, the stress is on the second syllable:

• si-RE-ne

A rough English-friendly guide:

• si = like see
• re = a tapped/flapped r, somewhat like the quick tt in American butter
• ne = like nee, but often lighter

So the main thing to remember is that the stress is on re: si-RE-ne.