Learn Finnish: Poika lipsautti oudon sanan ohimennen. - The boy let an odd word slip in passing.

This specific sentence actually does not use sattua. Instead, it uses a verb that inherently means to do something by accident. If you wanted to explicitly apply the lesson's grammar to this sentence, you would say Poika sattui lipsauttamaan oudon sanan ohimennen.

How does the verb lipsauttaa capture the idea of an accident or coincidence on its own?

The verb lipsauttaa means to let something slip out accidentally, such as a word or a secret. It is derived from lipsua (to slip). Even without the verb sattua, it expresses a sudden, unintentional action that perfectly fits the theme of chance and coincidence.

What grammatical structure is used to form the word ohimennen?

It is the second infinitive in the instructive case, formed from the phrasal verb mennä ohi (to go past). The -en ending on the second infinitive stem (menne-) tells you how an action was done. Here, it translates literally to "by going past," which gives us the English phrase "in passing."

Why are the words oudon sanan in the genitive case instead of the partitive?

They act as the total direct object of the sentence. Because the sentence has a standard active verb (lipsautti) and a clear subject (poika), a singular object must take the genitive-looking -n ending to show that the action (letting the whole word slip) was completed.