The distributive singular
In English, when a group of people put on their gear, we use a plural noun: "The divers put their wetsuits on." In Finnish, if each person in the group puts on exactly one item, the noun must be singular.
Using the plural in Finnish would actually imply that each diver put on several wetsuits! Because it is one wetsuit per person, Finnish uses the singular.
One item per person
This logic, known as the distributive singular, applies to any inherently individual item that each member of a group has exactly one of. It is commonly used for body parts, clothing, and extreme sports gear, whether the athletes are wearing, carrying, checking, or dropping them.
Object cases with the distributive singular
Even though the noun often translates as a plural in English, it is grammatically a singular object in Finnish. This means it strictly follows the normal rules for singular objects.
Most affirmative actions will put the object in the genitive case (ending in -n). However, if the sentence is negative, the singular object must be in the partitive case.
Likewise, if the sentence uses a necessity verb like täytyy (must), or is constructed in the colloquial passive (like me laitetaan, "we will put"), the object drops the -n and takes the nominative form, exactly as any other singular object would.