Learn Finnish: Tyttö on nyt kylvyssä. - The girl is in the bath now.

The basic dictionary word for "bath" is kylpy. When we add the "in" ending -ssä, it closes the final syllable. This triggers consonant gradation. For this word, the single p changes to v, so the stem kylpy- becomes kylvy- before we attach -ssä.

Why does kylvyssä end with -ssä instead of -ssa?

This happens because of vowel harmony, which you learned earlier. The root word kylpy contains the front vowel y. Because there are no back vowels (a, o, or u) in the word, the ending must also use a front vowel. That gives us -ssä.

The word tyttö has a double tt. Does it change because of consonant gradation too?

It can, but not in this exact sentence! Here, tyttö is just the subject in its basic dictionary form, so it stays "strong" with the double tt. If you added an ending that closes the syllable, like saying "in the girl" (tytössä), it would drop one t.

Does kylvyssä mean she is in the bathroom or literally inside the bathtub?