Learn Finnish: Keväällä me olemme koulussa, mutta kesällä me olemme puistossa. - In the spring we are at school, but in the summer we are in the park.

In English we say "in" a season, but Finnish uses the adessive case ending -lla or -llä, which usually means "on". So you are literally saying "on spring" and "on summer".

The dictionary word for spring is kevät. Why does it change so much in keväällä?

Finnish words ending in -t often change their shape before you add an ending. The -t drops out and the last vowel doubles, so kevät becomes the stem kevää-. Then you add the -llä ending.

If we use -llä for the seasons, why are we using -ssa for koulussa and puistossa?

While seasons use -lla or -llä, being physically inside a place usually requires the inessive ending -ssa or -ssä ("in" or "inside"). Finnish treats puisto (park) as a bounded area, so you are "inside" the park (puistossa), just like you are inside the school (koulussa).

Why do the endings look different: -llä with dots for the seasons, but -ssa without dots for the places?