Learn Finnish: Sukeltajat pudottivat suukappaleen huomaamattaan. - The divers dropped their mouthpieces without noticing.

This showcases the distributive singular, which is the focus of this lesson. Because each diver has exactly one mouthpiece, Finnish prefers the singular to express that they each dropped their own. English naturally switches to the plural here, but in Finnish, using the plural suukappaleet could imply that each diver dropped multiple mouthpieces.

Why does suukappaleen take the accusative ending -n instead of the partitive case?

Dropping something is a bounded, completed action. The mouthpieces did not just undergo a continuous process of dropping; they were completely dropped. Therefore, the object takes the total object case, which for singular nouns looks identical to the genitive.

How is the word huomaamattaan constructed?

It comes from the verb huomata (to notice). It combines the third infinitive abessive ending -matta (without doing) with the third-person possessive suffix -an (their). Together, it translates literally to without their noticing.

Could we use putosivat instead of pudottivat here?

No, because pudota (past tense 3rd person plural putosivat) is an intransitive verb meaning to fall. Since the divers are actively dropping a direct object, you must use the transitive verb pudottaa (past tense pudottivat), which means to drop.