Learn Swedish: Arkeologerna hade arbetat länge innan vasen blev rengjord. - The archaeologists had worked for a long time before the vase was cleaned.

In a dynamic passive sentence using bli, the past participle acts like an adjective and must agree with the subject. Because the subject here is vasen (a singular en-word), the participle takes the en-word form rengjord. If the subject were an ett-word like golvet (the floor), it would be rengjort, and if it were plural like skålarna (the bowls), it would be rengjorda.

Why does arbetat end in -t while rengjord ends in -d if they are both past forms?

They do different jobs in the sentence! The word arbetat is in the supine form, which is used after har or hade to create perfect tenses (hade arbetat). The supine never changes its ending. rengjord, however, is a past participle used with blev. Past participles change their endings to match the subject, just like adjectives do.

Could I use the s-passive here and say innan vasen rengjordes?

Yes, you absolutely could! Both blev rengjord and rengjordes translate to "was cleaned." The bli-passive emphasizes the result or the transition of the vase becoming clean, which is very common in spoken Swedish. The s-passive often sounds a bit more formal and focuses on the action itself.