Learn Finnish: Tuli tehtyä suuri virhe, minkä oivalsin myöhemmin. - A big mistake came to be made, which I realized later.

While your current lesson covers using mikä to refer back to superlatives, mikä is also the required pronoun when referring back to an entire preceding clause. Here, minkä refers to the whole situation—the fact that a mistake was made—rather than just the noun virhe.

Why is mikä in the form minkä?

It acts as the direct object of the verb oivaltaa (to realize). Because the action of realizing is complete, the total object takes the accusative case, which looks identical to the genitive form minkä.

What exactly is the tuli tehtyä construction?

This is a common structure used to express that an action happened accidentally or unintentionally. It is built using the verb tulla in the third-person singular (tuli) plus the partitive form of the passive past participle (tehtyä). It translates closely to ending up doing something or something happening to be done.

Since a mistake was made, why is suuri virhe in the nominative case instead of looking like an object?