Learn Finnish: Kunkin tuomarin olettamus oli väärä. - Each judge's assumption was wrong.

It is derived from the verb olettaa (to assume). The suffix -mus is added to the verb's strong stem to create a noun representing the result of the action, giving us the abstract concept of an assumption.

Where does the tricky form kunkin come from?

It is the genitive form of the pronoun kukin (each, every). When declining pronouns that end in the clitic -kin, the case ending goes in the middle, right before the clitic. The genitive of the root kuka is kun (an older variation of kenen), so kun + kin becomes kunkin.

Why are both kunkin and tuomarin in the genitive case?

They indicate who owns the assumption, translating to "each judge's". Since the pronoun kukin acts as a modifier for the noun tuomari, they must agree in case, so both take the genitive -n ending.

Could I use oletus instead of olettamus here?

Yes. Both words mean "assumption" and are derived from the same verb. However, olettamus sounds more formal, hypothetical, or legal, which fits perfectly with the context of a judge. Oletus is much more common in everyday speech and is also the word used for a "default" setting in software.