Learn Hardcore Portuguese (Portugal): A polícia não deixa ninguém sair do edifício. - The police do not let anyone leave the building.

Dropping the article (∅ polícia) is generally not correct in this kind of sentence. You might see polícia without an article in some set expressions (e.g. signs, headlines), but in normal speech and writing you say a polícia.

Why is the verb deixa in the singular if a polícia refers to many police officers?

In Portuguese, a polícia is grammatically a singular, feminine noun, even though it refers to a group. The verb always agrees with the grammatical subject, not with the “real-world” number of people.

So you say:

A polícia deixa… (3rd person singular, feminine)
A polícia chegou cedo. – The police arrived early.

If you explicitly refer to multiple officers, you use the plural:

Os polícias não deixam ninguém sair. – The (individual) police officers don’t let anyone leave.

What exactly does deixar mean here? Is it “leave” or “let”?

Deixar has several meanings. Here it is being used in the sense of “to allow / to let / to permit”:

deixar alguém fazer alguma coisa – to let/allow someone to do something

So:

A polícia não deixa ninguém sair…
literally: The police don’t let anyone leave…

The “to leave” part is expressed by sair, not by deixar, in this particular structure.

How does the structure deixar + alguém + infinitive work?

The pattern is:

deixar + direct object (person) + infinitive verb

It expresses permission (or lack of it):

Deixei o meu filho sair. – I let my son go out.
Ela não me deixa falar. – She doesn’t let me speak.
A polícia não deixa ninguém sair. – The police don’t let anyone leave.

Here:

deixa = verb “deixar”
ninguém = direct object (who is being allowed/not allowed)
sair = infinitive verb (what they are being allowed/not allowed to do)

Could I use permitir instead of deixar? What’s the difference?

Yes, you can, but the grammar changes slightly and the tone becomes more formal:

A polícia não deixa ninguém sair do edifício.
(everyday, neutral)
A polícia não permite que ninguém saia do edifício.
(more formal, legalistic)

With permitir, you typically use a “que” + subjunctive clause:

não permite que ninguém saia (…that anyone leave).

With deixar, you normally use an infinitive:

não deixa ninguém sair (…let anyone leave).

Why do we need não if we already have ninguém (a negative word)?

In European Portuguese, ninguém usually needs não before the verb to form a standard, grammatically correct negative sentence:

A polícia não deixa ninguém sair.

Literally: The police not let no one leave.

This “double” marking is normal in Portuguese and is not considered incorrect the way it would be in standard English. You can also place ninguém at the beginning:

Ninguém sai do edifício. – No one leaves the building.

In that pattern, you typically don’t add não, because ninguém is now the subject and already carries the negation.

What’s the difference between ninguém and something like nenhuma pessoa?

Ninguém is the usual, natural word for “nobody / no one / anyone (in negatives)”. It’s what you will use almost all the time.

A polícia não deixa ninguém sair.

Nenhuma pessoa literally means “no person”, but it’s heavier and less natural in most contexts:

A polícia não deixa nenhuma pessoa sair. – Grammatically fine, but sounds stiff or overly emphatic, more like legal language or very formal writing.

For everyday speech, stick with ninguém.

Why is the word order não deixa ninguém and not não ninguém deixa?

The normal word order is:

[subject] + [não] + [verb] + [object]

So:

A polícia (subject)
não (negation particle)
deixa (verb)
ninguém (object)

Gives: A polícia não deixa ninguém…

Putting ninguém before the verb (“não ninguém deixa”) is not standard in this structure. If you want ninguém earlier for emphasis, you usually make it the subject:

Ninguém pode sair do edifício. – Nobody can leave the building.

Why is it sair and not ir embora or another verb for “leave”?

Sair means “to go out / to exit / to leave a place (physically)” and is the normal verb for leaving a building, room, car, etc.

sair de casa – leave the house
sair do carro – get out of the car
sair do edifício – leave/exit the building

Ir embora is more like “go away / leave (generally)” and doesn’t focus on exiting a specific physical space, although in context it often overlaps.

In this sentence, because do edifício specifies a concrete place you’re exiting, sair de is the most natural choice.

Why is it do edifício and not de o edifício or just de edifício?

Portuguese almost always contracts de + o into do:

de + o edifício → do edifício
sair de + o edifício → sair do edifício

You generally can’t say de o edifício; the contracted form is mandatory in normal grammar.

You also wouldn’t usually drop the article here (∅ edifício) in a specific situation like this. You’re referring to a particular building (the one everyone is in), so you use o edifício → do edifício.

Is there any difference between edifício and prédio here?

In European Portuguese:

prédio is the most common everyday word for a building, especially a block of flats or a multi-storey building.
edifício is a bit more formal or technical, and you see it more in official language, documents, or news reports.

So:

A polícia não deixa ninguém sair do edifício.
A polícia não deixa ninguém sair do prédio.

Both are correct and mean the same here; the first just sounds slightly more formal or “written”.

Could I say A polícia não deixa sair ninguém do edifício? Is that wrong?

You can move ninguém a bit, but it changes the focus and some versions sound awkward.

A polícia não deixa ninguém sair do edifício. – neutral, most natural.
A polícia não deixa sair ninguém do edifício. – grammatically possible, but the default, neutral order is less this one; it can sound slightly marked (focus on the action “deixar sair”).

In normal speech and writing, you should prefer:

A polícia não deixa ninguém sair do edifício.

How would this sentence change if I wanted to specify a particular person instead of “anyone”?

You replace ninguém with a direct object pronoun or a noun phrase:

A polícia não o deixa sair do edifício. – The police don’t let him leave the building.
A polícia não a deixa sair do edifício. – …don’t let her leave…
A polícia não deixa o João sair do edifício. – …don’t let João leave…
A polícia não deixa os trabalhadores sair do edifício. – …don’t let the workers leave…

Note the pronoun position in European Portuguese in the negative:

não o deixa, não a deixa (pronoun before the verb because of não).

Why is the pronoun (if used) placed before deixa in the negative, but sometimes after the verb in other sentences?

In European Portuguese, object pronouns normally come after the verb in affirmative clauses:

A polícia deixa-o sair. – The police let him leave.

But with certain “trigger” words, including não, the pronoun is pulled before the verb:

A polícia não o deixa sair. – The police don’t let him leave.

So:

Affirmative: deixa-o
Negative: não o deixa

This rule is specific to European Portuguese; Brazilian Portuguese usually places pronouns before the verb in both cases (não deixa ele sair, não o deixa sair, etc.).