Learn Finnish: Poika juo vettä kylvyssä. - The boy drinks water in the bath.

Vesi is a mass noun—a substance you cannot easily count. When a mass noun is the object of an affirmative sentence (the thing being drank), it must take the partitive case to show the boy is drinking an indefinite amount ("some water"). The partitive form of vesi is vettä.

How does vesi become vettä? Where does the double tt come from?

Many older, native Finnish words that end in -si follow a special pattern. When you add an ending to them, the -si often changes to -te- or -tt-. For the partitive case, the stem of vesi becomes vett-, and we add the partitive ending -ä to get vettä.

The basic word for bath is kylpy. Why does it change to kylvy- in kylvyssä?

This happens because of a rule called consonant gradation. When we add the ending -ssä (meaning "in"), it closes off the final syllable. This "squeezes" the word and forces the strong consonant p in kylpy to weaken into a v, resulting in kylvyssä.

Why does kylvyssä use the -ssä ending instead of the regular -ssa for "in"?