Learn Finnish: Uusi kenkä ei mahdu pieneen laatikkoon. - The new shoe does not fit into the small box.

The verb mahtua (to fit) requires the illative case, which answers the question mihin (into where). In Finnish, a thing doesn't just "fit a box," it always fits into a box. To say "into the box", you take laatikko, double the last vowel, and add -n, making it laatikkoon.

Why is the word for "small" written as "pieneen" instead of "pieni"?

In Finnish, adjectives have to match the noun they describe. Since the box is in the illative case (laatikkoon), the adjective must also be in the illative case. The word pieni changes to its stem piene- before adding the illative ending -en, becoming pieneen.

The dictionary form of the verb is "mahtua". Where does the "d" in "mahdu" come from?

This happens because of a rule called consonant gradation. When the verb is made negative (ei mahdu), the strong ht in the middle of mahtua weakens into a hd. Also, because it follows the negative word ei, it drops its final personal ending.

I learned that negative sentences often use the partitive case. Why isn't "uusi kenkä" in the partitive?