Learn Finnish: Tiimalasi osoittautui tarpeelliseksi, koska se avasi pienen oven. - The hourglass turned out to be necessary because it opened a small door.

The verb osoittautua (to turn out to be) requires the word describing the outcome to be in the translative case. The translative case is often used to describe a change into a new state or role. Because the hourglass turned out to be necessary, tarpeellinen takes the translative ending -ksi.

How do we get from the base form tarpeellinen to the stem used in tarpeelliseksi?

Finnish words ending in -nen change their ending to -se- before adding most case suffixes. Therefore, the base adjective tarpeellinen drops the -nen for -se- to form the stem tarpeellise-. When you attach the translative suffix -ksi to this stem, you get tarpeelliseksi.

Why are pienen and oven in the genitive-like accusative case here instead of the partitive case?

They form a total object. The verb avata (to open) expresses a completed action here: the small door was successfully and entirely opened. Since the subordinate clause has a clear subject (se), the total object takes the accusative case that looks identical to the genitive, ending in -n.

What are the root words that make up the compound word tiimalasi?