Não se aceita nenhum cheque sem assinatura.

Here se is not reflexive. It is the impersonal/passive se often used in notices, rules, and signs.

So Não se aceita... means something like:

• ...is not accepted
• ...is not accepted here
• one does not accept...

In natural English, this sentence is best understood as a notice such as No unsigned checks accepted or Checks without a signature are not accepted.

Because the noun phrase after the verb is nenhum cheque, which is singular.

In this kind of passive-like structure, standard Portuguese usually makes the verb agree with that noun phrase:

• Aceita-se nenhum cheque would not be normal because of the negation issue, but the agreement idea is the same.
• Não se aceita nenhum cheque... → singular noun, so aceita
• Não se aceitam cheques... → plural noun, so aceitam

So aceita matches nenhum cheque.

Because nenhum normally goes with a singular noun and means no / not a single.

So:

• nenhum cheque = no check / not a single check

Even though English often uses a plural noun after no, Portuguese usually uses the singular after nenhum.

The plural forms nenhuns / nenhumas do exist, but they are rare and usually not used in everyday sentences like this one.

In Portuguese, this is normal and correct. Portuguese uses negative concord, which means multiple negative words can appear in the same sentence while expressing just one negative idea.

So:

• Não se aceita nenhum cheque...

does not mean a positive statement. It simply means:

• No check is accepted...
• Not a single check is accepted...

This is very common in Portuguese:

• Não vi ninguém. = I didn’t see anyone.
• Não tenho nada. = I don’t have anything.

Because after sem, Portuguese often omits the article when speaking generally.

So:

• sem assinatura = without a signature

This means the check lacks the required signature in general.

If you said sem a assinatura, that would usually sound more specific, like without the signature of some particular person or the specific signature expected in that context.

Yes. That is perfectly natural.

It means:

• We do not accept checks without a signature

The difference is mostly in style:

• Não se aceita nenhum cheque sem assinatura → more impersonal, more like a formal notice or sign
• Não aceitamos cheques sem assinatura → more direct, as if the store or business is speaking

Both are correct and natural.

Yes, absolutely.

That version uses a plural noun:

• cheques

So the verb also goes plural:

• aceitam

Compare:

• Não se aceita nenhum cheque sem assinatura = No check without a signature is accepted / Not a single unsigned check is accepted
• Não se aceitam cheques sem assinatura = Checks without a signature are not accepted

The meaning is very similar. The version with nenhum cheque sounds a little more emphatic: not a single check.

Yes. This kind of structure is very common in formal written notices, rules, and signs.

Examples of the same style:

• Não se aceita cartão.
• Não se vendem bebidas alcoólicas para menores.
• Não se troca mercadoria em promoção.

It sounds official, neutral, and impersonal, which is why businesses often use it.

Yes. A similar idea could be expressed with an adjective, for example:

• Não se aceitam cheques não assinados.

That means Unsigned checks are not accepted.

Both are natural, but they feel slightly different:

• sem assinatura = without a signature
• não assinados = not signed

In a notice, sem assinatura is very common and straightforward.

Cheque is pronounced roughly like SHEH-kee in Brazilian Portuguese.

More exactly, it is usually:

• /ˈʃɛ.ki/

So:

• ch sounds like sh
• the final que sounds like kee

Also, cheque is the normal Brazilian Portuguese word for check in the banking/payment sense, even though checks are less common than they used to be.