Quando ouvimos a sirene, a motorista diminuiu a velocidade imediatamente.

Ouvimos can be either present tense (we hear) or simple past / preterite (we heard). In this sentence, it is understood as preterite because the other main verb, diminuiu, is also in the past, and the sentence describes a completed event.

So here:

• quando ouvimos a sirene = when we heard the siren

This kind of ambiguity is normal with some nós verb forms in Portuguese.

Yes, by itself it can. For the verb ouvir:

• nós ouvimos = we hear
• nós ouvimos = we heard

The form is identical in both tenses. Context tells you which one is meant. In this sentence, the past reading is the natural one.

Quando means when and introduces a time clause.

Here it sets the moment that triggered the next action:

• Quando ouvimos a sirene... = When we heard the siren...

It is very common to use quando with past actions like this.

Because the sentence begins with a subordinate clause:

• Quando ouvimos a sirene, ...

In Portuguese, when a clause like this comes first, a comma is normally used before the main clause. It works much like English:

• When we heard the siren, the driver slowed down immediately.

If the main clause came first, the comma might not be necessary:

• A motorista diminuiu a velocidade imediatamente quando ouvimos a sirene.

Motorista is a noun of common gender in Portuguese. That means the noun form stays the same, and the article shows whether the person is male or female:

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

So even though it ends in -a, that does not automatically make it feminine in form. The article is what tells you the gender here.

Portuguese uses definite articles (o, a, os, as) more often than English does.

So Portuguese naturally says:

• ouvimos a sirene
• diminuiu a velocidade

Even when English might say:

• we heard the siren
• slowed down

or sometimes just reduced speed without sounding as article-heavy.

In Portuguese, these articles are completely natural and expected.

Diminuir a velocidade is a very common expression meaning to reduce speed / to slow down.

Literally:

• diminuiu = reduced
• a velocidade = the speed

So the whole expression works idiomatically as:

• she slowed down

Portuguese can also use verbs like desacelerar, but diminuir a velocidade is extremely natural and common in everyday language.

Diminuiu is the preterite, which is used for a completed action:

• the driver heard or reacted to the siren and then slowed down

Diminuía is the imperfect, which would suggest an ongoing, repeated, or background action:

• A motorista diminuía a velocidade... could sound like the driver was slowing down / used to slow down

In this sentence, the action is a specific completed event, so diminuiu is the right choice.

Adverb placement in Portuguese is fairly flexible, and putting imediatamente at the end sounds very natural.

So:

• A motorista diminuiu a velocidade imediatamente.

means the same as:

• A motorista imediatamente diminuiu a velocidade.
• Imediatamente, a motorista diminuiu a velocidade.

The end position is often the most neutral and common in everyday sentences.

Yes. Portuguese allows some flexibility in word order, especially with adverbs and subordinate clauses.

For example:

• A motorista diminuiu a velocidade imediatamente quando ouvimos a sirene.
• Quando ouvimos a sirene, a motorista imediatamente diminuiu a velocidade.

These are all understandable, but the original version is very natural and well balanced.

Yes, escutar could also work in many contexts.

Very roughly:

• ouvir = to hear
• escutar = to listen

But in real Brazilian Portuguese, the distinction is not always strict, and people often use ouvir in places where English would use either hear or sometimes even listen to.

So:

• Quando ouvimos a sirene... is completely natural.

Diminuiu can be tricky for English speakers because of the vowel sequence.

It comes from:

• infinitive: diminuir
• preterite: diminuiu = he/she reduced

In Brazilian Portuguese, it is pronounced roughly like:

• jee-mee-noo-EE-oos? Not exactly — that rough English approximation is imperfect.

A better breakdown is:

• di-mi-nu-iu

The final part -iu is common in verbs like this:

• construiu
• decidiu (different pattern, but similar challenge for learners)

The key is not to over-separate it into too many syllables.

Yes, it works perfectly in Brazilian Portuguese, and it is also standard Portuguese more generally.

Nothing in the sentence is unusual or slangy. It is a normal, standard sentence that a Brazilian speaker would easily use and understand.