Learn Finnish: Tämä kampi on tarpeellinen seuraavassa huoneessa. - This crank is necessary in the next room.

Since the current lesson is about the verb osoittautua (to turn out to be), how would we change this sentence to say 'This crank turns out to be necessary...'?

You would replace the verb on with osoittautuu and change the adjective tarpeellinen into the translative case, which expresses a change or outcome. The sentence would become: Tämä kampi osoittautuu tarpeelliseksi seuraavassa huoneessa.

What is the difference between saying on tarpeellinen and on tarpeen?

While both translate to 'is necessary' or 'is needed', tarpeellinen is an adjective describing the inherent quality of the crank as a useful or necessary object. Tarpeen is an idiomatic use of the illative form of tarve (need) and usually means something is currently required or 'needed' in the moment.

Why does seuraava (next) take the -ssa ending here?

In Finnish, modifiers like adjectives must agree in case with the noun they modify. Because huone is in the inessive case (huoneessa, 'in the room'), the adjective seuraava must also be in the inessive case, becoming seuraavassa.

As a quick refresher, why does huone become huoneessa with a double 'e'?