Learn Finnish: Äiti luki kirjaa lastenhuoneessa. - Mother read a book in the children's room.

In Finnish compound words, the first part is often in the genitive case when it expresses the purpose or target user of the second part. Since the room is meant specifically for children, lapsi becomes the genitive plural lasten. If the first word merely described a physical material or basic type, it would usually stay in the nominative case.

Are there other common words that follow this genitive purpose rule like lastenhuoneessa does?

Yes! You will see this pattern frequently when an item or concept is designed for a specific group of people or animals. Common examples include naistenpyörä (women's bicycle), lastenlääkäri (pediatrician, literally children's doctor), and koiranruoka (dog food, where koira takes the singular genitive koiran).

Why is the object kirjaa in the partitive case here instead of the genitive-accusative kirjan?

Using the partitive kirjaa indicates that the action of reading was an ongoing or incomplete process. It essentially means Mother was reading a book or read some of a book. If the sentence used the genitive-accusative kirjan, it would imply a completed action where she read and finished the entire book.

Why do we use the inessive ending -ssa for lastenhuoneessa instead of the adessive -lla?