Learn Finnish: Pelaaja osoittautuu kekseliääksi pakohuoneessa. - The player turns out to be inventive in the escape room.

This sentence uses the verb osoittautua (to turn out to be). In Finnish, verbs describing an outcome, transformation, or something becoming known usually require the following noun or adjective to be in the translative case, which ends in -ksi.

How is the stem kekseliää- formed from kekseliäs?

For adjectives ending in -as or -äs, the inflectional stem is formed by dropping the final -s and lengthening the preceding vowel. So, kekseliäs becomes the stem kekseliää-, to which the translative ending -ksi is attached.

Is the verb osoittautua related to the verb osoittaa (to show or to point)?

Yes, it is formed by adding the automative or reflexive suffix -utua to the stem of osoittaa. While osoittaa means 'to show', the reflexive osoittautua essentially means 'to show itself as' or 'to turn out to be'.

Are there other common verbs that work like osoittautua and trigger the translative case?

Yes! Another very common one is tulla when used to mean 'to become' (for example, hän tuli sairaaksi meaning 'he became sick'). Verbs expressing a change of state, such as muuttua (to change into) or valmistua (to graduate as), also require the -ksi ending.