Learn Finnish: Hän huomasi valituksen unohdetun. - He noticed the complaint had been forgotten.

This sentence uses the past passive referative construction (partisiippirakenne). Instead of using an että clause, the verb becomes a participle. The past passive participle of unohtaa is unohdettu. We add the genitive -n because the whole phrase valituksen unohdetun acts as the object of the main verb huomasi.

Why is the word for complaint in the genitive case (valituksen)?

In this participial construction, the subject of the action (the thing being forgotten) must be put into the genitive case. Therefore, the basic word valitus (complaint) changes to valituksen.

How would I say this using a normal että (that) clause?

You would say: Hän huomasi, että valitus oli unohdettu. The referative construction used in our example sentence means exactly the same thing but is shorter and much more common in written or formal Finnish.

How do we know to use the passive participle (unohdetun) instead of the active one (unohtaneen)?

We use the passive because a complaint cannot forget things; it was forgotten by an unspecified person. If the sentence was about a specific person forgetting something, we would use the active past participle unohtaneen instead.