Learn Finnish: Tämä laite on täysin hyödytön, joten se on palautettava. - This device is completely useless, so it must be returned.

The word palautettava is the passive present participle of palauttaa (to return something). On its own, this participle roughly means "to be returned". When paired with the verb olla (to be), it creates a necessity structure. So, on palautettava literally means "is to-be-returned", which is a very common way to say "must be returned" in Finnish without using verbs like täytyy or pitää.

I learned that necessity sentences usually take a genitive subject. Why is there no genitive word here?

This sentence is a passive statement where the person doing the action isn't specified. If you wanted to say "I must return it", you would add a genitive subject: Minun on palautettava se. Because this sentence just means the action must be done generally, there is no genitive subject.

What is the difference between the verbs "palauttaa" and "palata"?

Both translate to "to return" in English, but they are used differently. Palata means returning oneself to a place (like returning home or returning to work). Palauttaa means returning an object to someone or somewhere (like returning a faulty device to a store). Since a device is being returned here, we must use palauttaa.

How is the word "hyödytön" (useless) built?