Learn Finnish: Poika piiloutuu pensaikkoon, mutta tyttö juoksee kivikkoon. - The boy hides in the thicket, but the girl runs into the rocky area.

The base word is kivi (stone or rock). Adding the cluster suffix -kko creates kivikko, meaning a group of rocks, a rocky patch, or a boulder field. The -on ending is the illative case (into).

What is the base word for pensaikko, and how does its stem change?

The base word is pensas (bush or shrub). Its vowel stem is pensaa-. When we add the cluster suffix -ikko to describe a group of bushes (a thicket), the double aa drops one a to accommodate the i, resulting in pensaikko.

Why are both pensaikkoon and kivikkoon in the illative (into) case?

In Finnish, hiding and running to a location are seen as movements towards a destination. Both the verb piiloutua (to hide oneself) and juosta (to run) answer the question mihin? (where to?), which requires the illative case ending -on.

The verb piiloutuu comes from the noun piilo (hiding place). What does the -utua ending do here?

The suffix -utua (or -ytyä) often creates reflexive or automative verbs. It transforms the noun piilo into the verb piiloutua, meaning "to put oneself into a hiding place" or simply "to hide oneself".