Learn Dutch: De buitenaardse bezoeker zou haar hebben ontvoerd. - The extraterrestrial visitor supposedly abducted her.

This is the evidential use of zouden. By combining the past tense zou with a perfect infinitive (hebben ontvoerd), the speaker indicates that the event reportedly happened in the past, but they are not guaranteeing it is a fact. In English, we usually express this with words like 'supposedly', 'allegedly', or 'is said to have'.

Could I say ontvoerd hebben instead of hebben ontvoerd at the end of the sentence?

Yes, you can. In a Dutch verbal cluster at the end of a clause, the auxiliary verb (hebben) can go either before or after the past participle (ontvoerd). Both options are grammatically correct and mean the exact same thing, though placing the auxiliary first is slightly more common in written Dutch.

Why isn't there an explicit adverb for 'supposedly' in the Dutch sentence?

Because the concept of 'supposedly' is already built into the grammar itself. The verb zouden takes on this 'hearsay' meaning when used as an auxiliary verb in this way. Adding a word like zogenaamd (supposedly) or beweerdelijk (allegedly) is not strictly necessary and would often feel redundant here.

How is the adjective buitenaardse formed?