Learn Swedish: Vilka föremål blev upptäckta igår? - Which objects were discovered yesterday?

In this type of passive sentence, the past participle acts like an adjective describing the subject. Because the subject föremål is plural here, the participle must take the plural -a ending, changing from upptäckt to upptäckta.

How do we know föremål is plural if it doesn't have a plural ending?

Föremål is an ett-word that ends in a consonant. These words look exactly the same in both singular and plural indefinite forms. You can tell it is plural in this sentence because it follows the plural question word Vilka and takes the plural participle upptäckta.

If there was only one object, how would this sentence change?

You would change the question word to the singular ett-word form Vilket, and you would remove the plural -a ending from the participle so it agrees with a singular ett-word. The sentence would become Vilket föremål blev upptäckt igår?

Could I use the s-passive and say Vilka föremål upptäcktes igår?

Yes, both versions are completely correct and mean the same thing. Using blev plus the past participle often sounds a bit more conversational and emphasizes the sudden event of being discovered, while the s-passive is slightly more common in written text or formal reports.