Learn Finnish: Töissä tuli tehtyä iso moka. - I accidentally made a big blunder at work.

This is the inadvertent action construction. It uses the 3rd person singular past tense tuli (it came) plus the passive past participle in the partitive case, tehtyä (to be done). Together, they express that an action happened accidentally, thoughtlessly, or without full intention.

Why is there no word for "I" in the Finnish sentence?

In this construction, the person who made the mistake is often left out if it is obvious from the context, keeping the focus on the accident itself. If you needed to specify who did it, you would use the adessive case: Minulla tuli tehtyä iso moka.

Why is the object iso moka in the nominative case instead of ison mokan?

The structure tuli tehtyä acts as an impersonal, passive-like construction. Because there is no formal grammatical subject (like "minä") performing the action, a total object defaults to the nominative case (iso moka) rather than the genitive-looking accusative.

Is moka a standard written word for "mistake"?

No, moka is a very common spoken-language or colloquial word for a blunder or slip-up. In formal, standard Finnish, you would use a word like virhe or erehdys.