Questions & Answers about Miksi sinä hyppäät veteen?
Why do we use veteen (into the water) instead of vedessä (in the water)?
This is about verb rection! The verb hypätä (to jump) is an action of movement. In Finnish, when an action involves moving from the outside to the inside of a place, we use the illative case for 'into' (veteen). If you used vedessä, it would mean you are already inside the water and just jumping up and down while staying there.
How does the word vesi (water) turn into veteen?
Old Finnish words ending in -si have a special stem. The -si changes to -te-, giving us the base vete-. To say 'into', we add the illative case ending. For short words ending in a vowel, you double the last vowel and add -n, making it veteen.
The dictionary form of the verb is hypätä, so why are there two p's in hyppäät?
This is an example of consonant gradation. Hypätä belongs to a verb group where the dictionary form has a 'weak' consonant (one p). When you conjugate it to say who is doing the action, the stem gets 'stronger' and the single p doubles to pp, making the stem hyppää-.
Is the word sinä (you) required in this sentence?
No, it is optional. Because the verb ending -t in hyppäät already tells us exactly who is doing the action ('you'), you can safely drop sinä. Saying Miksi hyppäät veteen? is very common and natural in spoken Finnish.