Learn Finnish: Kissa nukkuu sängyssä. - The cat sleeps in the bed.

The verb nukkua (to sleep) is a "static" verb. It describes an action where you stay in one place, rather than moving to or from a location. Because the cat's location doesn't change, we use the static "in" case (answering the question "where?"), which adds -ssä to the word.

The dictionary word for bed is sänky. Why does it change to sängy- in sängyssä?

This happens because of a spelling rule called consonant gradation. When you add an ending like -ssä, the strong consonant cluster nk in the middle of sänky weakens to ng. Therefore, sänky becomes sängyssä.

Why is the ending -ssä and not -ssa?

This is due to vowel harmony. The word sänky contains the front vowels ä and y. Finnish rules say that if a word has front vowels, its endings must also use front vowels, so we use -ssä instead of the back-vowel version -ssa.

Why does the verb nukkuu end with a double u?