Não deves acelerar numa rua tão estreita, especialmente quando está a chover.

Because deves is the 2nd person singular form of dever, used with tu:

• (tu) deves = you should / you must
• (você / ele / ela) deve = you should / he/she should

So this sentence is speaking informally to one person:

• Não deves acelerar... = You shouldn’t speed...

If you wanted the more formal you, you would say:

• Não deve acelerar...

Here, dever expresses advice, recommendation, or a mild sense of obligation.

So Não deves acelerar means something like:

• You shouldn’t speed
• You mustn’t speed depending on context, though here shouldn’t is usually the best match

In Portuguese, dever + infinitive is very common for saying what someone should do:

• Deves estudar. = You should study.
• Não deves fumar. = You shouldn’t smoke.

In Portuguese, não normally goes directly before the verb it negates.

So:

• Não deves acelerar = You shouldn’t speed

That is the normal word order. Compare:

• Deves parar. = You should stop.
• Não deves parar aqui. = You shouldn’t stop here.

English learners sometimes want to place the negative elsewhere, but in Portuguese, putting não before the verb is the standard pattern.

Because after dever, Portuguese normally uses an infinitive.

The structure is:

• dever + infinitive

So:

• deves acelerar = you should speed
• deves esperar = you should wait
• não deves conduzir = you shouldn’t drive

This is similar to English modal verbs like should, which are also followed by a base verb.

Numa is the contraction of:

• em + uma = numa

So:

• numa rua = in a street / on a street

These contractions are very common and usually expected in normal Portuguese:

• em + um = num
• em + uma = numa
• em + uns = nuns
• em + umas = numas

So numa rua tão estreita is the natural form.

Because estreita is an adjective agreeing with rua, and rua is feminine singular.

• rua = feminine singular
• therefore estreita = feminine singular form of estreito

Compare:

• um carro estreito = a narrow car
• uma rua estreita = a narrow street

Adjectives in Portuguese often change form to match the noun’s gender and number.

Tão means so in the sense of to such a degree.

So:

• uma rua tão estreita = such a narrow street / a street so narrow

Common pattern:

• tão + adjective

Examples:

• tão grande = so big
• tão difícil = so difficult
• tão estreita = so narrow

It is often used when the speaker wants to emphasize the quality.

Both are possible, but they are not exactly the same.

• quando está a chover = when it is raining
• quando chove = when it rains

In European Portuguese, estar a + infinitive is the normal way to express an action in progress. So está a chover focuses on the rain as something happening at that moment.

That makes it very natural here:

• especially when it is raining

If you said quando chove, it would sound more general:

• especially when it rains

Both can work, but está a chover matches the ongoing situation more closely.

Yes. In European Portuguese, the usual progressive form is:

• estar a + infinitive

So:

• está a chover = is raining
• está a trabalhar = is working
• está a falar = is speaking

In Brazilian Portuguese, the more common form is:

• estar + gerund

So a Brazilian would more naturally say:

• está chovendo
• está trabalhando
• está falando

Since your sentence is Portuguese from Portugal, está a chover is exactly what you would expect.

There is no expressed subject here. Portuguese often leaves out the subject when it is not needed.

With weather verbs like chover, English uses it:

• it is raining

But Portuguese usually does not use an equivalent dummy subject. So:

• está a chover literally behaves more like is raining

This is completely normal with weather expressions:

• Chove. = It rains / It’s raining
• Está a chover. = It is raining
• Nevou ontem. = It snowed yesterday

The comma separates the main warning from an added explanatory detail.

Main idea:

• Não deves acelerar numa rua tão estreita

Added emphasis:

• especialmente quando está a chover

In English, you would often do the same:

• You shouldn’t speed on such a narrow street, especially when it’s raining.

The comma helps the sentence read more naturally, though punctuation can sometimes vary depending on style.

Yes, and that is a very common alternative.

• Não deves acelerar = You shouldn’t speed / firmer, more direct advice
• Não devias acelerar = You shouldn’t be speeding / You shouldn’t speed / often softer, more tentative

In European Portuguese, the imperfect form devias is often used to soften advice.

So both are correct, but the tone changes slightly:

• Não deves... = more direct
• Não devias... = a bit gentler

Not with exactly the same meaning.

• numa rua = on/in a street, meaning on such a kind of street or on a certain narrow street
• na rua = on the street / in the street, referring to a specific street already known from context

So in this sentence:

• numa rua tão estreita = on such a narrow street

If you said na rua tão estreita, it would sound incomplete unless a specific street had already been identified, and even then it would usually need a fuller structure.

Yes, especialmente means especially, and sobretudo can often also mean especially / above all.

So these are both possible:

• especialmente quando está a chover
• sobretudo quando está a chover

Especialmente is a very direct match for English especially.
Sobretudo can sound slightly more like above all or particularly depending on context.

In this sentence, especialmente is very natural.

Yes, acelerar is a natural choice. It literally means to accelerate, but in traffic contexts it can also mean to drive too fast / speed depending on the sentence.

So:

• Não deves acelerar = You shouldn’t speed / accelerate

If you wanted to be even more explicitly about speed limits or excessive speed, Portuguese might also use expressions like:

• andar depressa demais
• exceder a velocidade
• ir rápido demais

But acelerar is perfectly idiomatic here.

If you are speaking to more than one person informally, European Portuguese often avoids a direct you plural verb form in everyday speech and may rephrase with vocês.

A natural version would be:

• Não devem acelerar numa rua tão estreita, especialmente quando está a chover.

Here:

• devem = you should for vocês

The rest of the sentence stays the same.

In European Portuguese, não is roughly pronounced like nowng, with a nasal sound. The exact quality is hard to represent in English.

A few tips:

• the ão ending is nasal
• it is not pronounced like a simple English ow
• do not pronounce a clear final n

So the important thing is to produce a nasal vowel rather than saying something like nao with two separate sounds.

Yes, it is clearly in the tu form because of deves.

In Portugal, tu is very common in many everyday situations, especially with:

• family
• friends
• children
• classmates
• people of similar age or in informal contexts

A more formal version would be:

• Não deve acelerar numa rua tão estreita, especialmente quando está a chover.

So the sentence sounds natural and normal for informal European Portuguese.