Question 1

Why is the modal verb dever in the plural (devem) here?

Accepted Answer

This sentence uses the passive se. The grammatical subject of the sentence is actually the plural noun as regras (the rules) — they are the things receiving the action. Because the subject is plural, the modal verb dever must also be plural (devem) to agree with it.

Question 2

Why does the pronoun come before the verb (se devem) instead of after (devem-se)?

Accepted Answer

The negative word Não is a "magnet" word. In European Portuguese, negative words at the beginning of a clause force the pronoun to move in front of the conjugated verb. Without the Não, you would attach the pronoun to the modal verb: Devem-se respeitar as regras.

Question 3

The English translation uses the passive voice ("must the rules be respected"). Can this also translate to an active, impersonal sentence?

Accepted Answer

Yes! The passive se is frequently used to express the impersonal idea of "one must" or "people must" (e.g., "Not only must one respect the rules..."). Just remember that even if you think of it as an active "one must respect" in English, Portuguese grammatically treats the things being respected (as regras) as the subject, which is why the verb is plural.

Question 4

How does the second half of the sentence work without a verb?

Accepted Answer

The structure Não só... mas também means "Not only... but also". Because both halves of the sentence share the exact same action, you can drop the verb in the second part to avoid repetition. The verbal phrase se devem respeitar is implicitly understood right before as restrições.

Não só se devem respeitar as regras, mas também as restrições.

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