Learn Finnish: Pieni moka tapahtui täysin huomaamatta. - A small blunder happened completely without noticing.

The tuli tehtyä structure requires an active agent who accidentally did something (for example, "I accidentally made a mistake" would be Minun tuli tehtyä moka). In this sentence, the subject is the mistake itself (Pieni moka), and the verb is tapahtui (happened). Because the sentence simply states that the event occurred, rather than focusing on someone accidentally doing it, the standard past tense is used.

What verb form is huomaamatta, and how is it used here?

It is the third infinitive abessive (the -matta or -mättä form) of the verb huomata (to notice). It translates directly to "without noticing." At this level, you will frequently see this form used to describe an action happening without an accompanying action taking place.

Is moka an everyday word, or is it slang?

Moka is a colloquial but extremely common word meaning "mistake" or "blunder." You will hear it constantly in spoken Finnish and see it in casual writing. The standard, more formal equivalent is virhe. You might also recognize the related verb mokata (to mess up).

Why does the sentence use täysin instead of täydellisesti for "completely"?

While both can translate to "completely," täysin acts as an intensifier for the following word, much like "totally" or "entirely" (here, emphasizing how unnoticed the mistake was). Täydellisesti is an adverb of manner that usually means "perfectly" or "flawlessly," which would sound strange when describing a blunder.