Learn Dutch: Burgers zouden hun plicht niet moeten vergeten. - Citizens should not forget their duty.

While moeten means "must" or "have to", combining it with zouden creates the meaning of "should" or "ought to". It turns a strict command ("Citizens must not forget") into strong advice or a moral obligation ("Citizens should not forget").

Why are there two infinitives (moeten and vergeten) at the end of the sentence?

In a main clause, the conjugated verb (zouden) takes the second position. Any other verbs are pushed to the very end. Because zouden moeten (should) and vergeten (forget) work together here, both infinitives stack up at the end of the sentence.

Why is niet placed after hun plicht rather than before it?

In Dutch, niet usually comes after a definite object. Because hun plicht includes a possessive pronoun (hun), it is a specific, definite object. Therefore, niet is placed after it, right before the verbs at the end.

What would this sentence look like if we were talking about a single citizen?

It would be: De burger zou zijn plicht niet moeten vergeten. Because the subject is now singular, the conjugated verb changes from zouden to zou, and the possessive pronoun changes from hun (their) to zijn (his/its) or haar (her).