Sem o cinto, a policial pode parar a motorista no cruzamento.

Sem means without.

Cinto literally means belt, but in a traffic/driving context it is very commonly understood as seat belt. A fuller, more explicit phrase would be cinto de segurança.

So in this sentence, sem o cinto is naturally understood as without the seat belt or not wearing a seat belt.

Because sem o cinto is an introductory phrase placed before the main clause.

The comma helps separate:

• the introductory circumstance: Sem o cinto
• from the main statement: a policial pode parar a motorista no cruzamento

In Portuguese, this comma is very common and natural. With a short introductory phrase, some commas are optional in practice, but here the comma makes the sentence clearer and smoother.

Grammatically, the sentence is ambiguous.

Sem o cinto could in theory describe:

• a policial — the police officer
• or a motorista — the driver

In real-world context, listeners will probably assume it refers to the driver, because that makes more sense in a traffic situation. But the grammar itself does not force that reading.

If you want to make it clearer, Portuguese would usually be rewritten, for example:

• Se a motorista estiver sem cinto, a policial pode pará-la no cruzamento.
• A policial pode parar a motorista, se ela estiver sem cinto.

Both words here are referring to women, and the feminine is shown by the article a.

But the two nouns behave a little differently:

• policial is a common-gender noun:

  • o policial = the male police officer
  • a policial = the female police officer

• motorista is also a common-gender noun:

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

So the noun itself does not change form here. The article tells you the gender.

Portuguese uses definite articles much more often than English does.

So a policial and a motorista are perfectly normal ways to say:

• the police officer
• the driver

In Portuguese, professions and roles are often introduced with articles when talking about a specific person in the situation.

This is a very common structure in Portuguese:

• pode = conjugated form of poder
• parar = infinitive

Together, pode parar means can stop, may stop, or is able/allowed to stop, depending on context.

This works like English modal-style verb combinations:

• can stop
• may stop

So only the first verb is conjugated, and the second stays in the infinitive.

It depends on context.

Pode can express:

• ability — can
• permission/authority — may / is allowed to
• possibility — might / could

In this sentence, because it involves a police officer and a driver, permission/authority is probably the most natural reading:

• the police officer may/can stop the driver

But in English, can is often the most natural translation even when authority is meant.

The normal reading comes from word order and verb structure.

The sentence pattern is:

• a policial = subject
• pode parar = verb phrase
• a motorista = object
• no cruzamento = location

So the most natural interpretation is:

• the police officer does the action
• the driver receives the action

If you reversed the nouns, the meaning would normally reverse too:

• Sem o cinto, a motorista pode parar a policial no cruzamento.

That would mean the driver can stop the police officer at the intersection, which is a very different sentence.

Because here parar is being used as a transitive verb meaning to stop someone or to pull someone over.

So:

• parar a motorista = to stop the driver

Portuguese does not use a special personal a the way Spanish often does. So the direct object can simply appear right after the verb:

• parar o carro = stop the car
• parar a motorista = stop the driver

Do not confuse this a with a preposition here. In a motorista, it is just the feminine definite article the.

No cruzamento means at the intersection or in the intersection, depending on context.

No is a contraction:

• em + o = no

Because cruzamento is masculine:

• o cruzamento = the intersection
• em o cruzamento becomes no cruzamento

This kind of contraction is extremely common in Portuguese:

• na = em + a
• nos = em + os
• nas = em + as

It is understandable, but many native speakers would probably want to make it less ambiguous.

The main issue is that Sem o cinto does not clearly say who is not wearing the seat belt.

A more natural, clearer version might be:

• Sem cinto, a motorista pode ser parada pela policial no cruzamento.
• Se a motorista estiver sem cinto, a policial pode pará-la no cruzamento.
• A policial pode parar a motorista no cruzamento se ela estiver sem cinto.

So the original sentence is grammatically possible, but in real use, Brazilians would often choose a clearer version.