Learn Finnish: Lapsi rakastaa lunta.

Because of the verb! The verb rakastaa (to love) is a "partitive verb." This means the thing being loved must always be in the partitive case. Therefore, the basic word lumi changes to its partitive form, lunta.

How does the spelling change so much from lumi to lunta?

Lumi is an older Finnish nature word, and its partitive form is a bit irregular. When the partitive ending -ta is attached, the m changes to an n to make it easier to say, resulting in lunta. It's best to just memorize this specific word, as you will use it a lot!

This lesson is about season cases like -lla (e.g., talvella). Why don't we use one of those endings here?

Cases like -lla or -llä are used to say when something happens, like "in winter" (talvella). In this sentence, the snow isn't a time period; it is the actual thing the child loves (the object). Because of this, its case is controlled by the verb rakastaa, not by the season.

How does the verb rakastaa show that "the child" is doing the loving?