Questions & Answers about Le loup est plus dangereux qu'ils ne croyaient.
Why is there a ne before croyaient? The English translation doesn't say "didn't believe."
This is the ne explétif. It doesn't make the sentence negative, which is why there is no pas. In French, it is used after an affirmative comparison of inequality (like plus... que or moins... que) to emphasize the gap between reality (the wolf is dangerous) and expectation (what they believed).
Is the ne explétif strictly required in this type of sentence?
It is highly recommended in written and formal French, but it is essentially optional. In everyday spoken French, it is very common to drop it entirely and just say plus dangereux qu'ils croyaient. Also note that it's only used if the main clause is affirmative; if the sentence were negative (Le loup n'est pas plus dangereux...), you wouldn't use the ne explétif at all.
Why use the imperfect tense croyaient instead of the passé composé ont cru?
Do I pronounce the 's' at the end of plus in this sentence?