Learn Finnish: Minä katsoin, kun lintu lensi linnunpönttöön. - I watched when the bird flew into the birdhouse.

This perfectly illustrates our current lesson! A linnunpönttö is a nest box (pönttö) meant for a bird. When the first part of a compound noun expresses the purpose, target, or intended user of the second part, it takes the genitive case (linnun).

I learned that katsoa (to watch) takes the partitive case. Why isn't it katsoin lintua here?

You are right that katsoa normally requires a partitive object, like katsoin lintua (I watched the bird). However, in this sentence, you aren't just watching a noun; you are watching an entire action unfold. The subordinate clause starting with kun (when) acts as the object of what you are watching, so no partitive noun is needed in the main clause.

What case is linnunpönttöön, and how is it formed from the base word?

It is in the illative case, which expresses movement into something (into the birdhouse). To form the illative, you take the base word linnunpönttö, double the final vowel (ö), and add -n, resulting in linnunpönttöön.

The verb form is lensi. Why doesn't it look more like its dictionary form lentää?