Learn Finnish: Lapsi syö makeaa suklaata. - The child is eating sweet chocolate.

This is adjective agreement in action! In Finnish, adjectives have to match the case of the noun they describe. Because the noun suklaata (chocolate) is in the partitive case, the adjective makea (sweet) must also take a partitive ending, becoming makeaa.

Both words are in the partitive case, so why does makeaa end in -a but suklaata ends in -ta?

The partitive ending changes depending on the word's final letters. The basic word makea ends in a single short vowel, so it just gets an extra -a. However, the basic word suklaa ends in a double vowel, and words ending in two vowels always take the -ta or -tä ending.

Why do we need the partitive case for sweet chocolate in the first place?

We use the partitive because chocolate is an uncountable mass noun (like water or rice), and the child is eating an unspecified amount of it (some sweet chocolate). When eating or drinking something uncountable, the object must be in the partitive case.

The English translation says is eating. Does syö specifically mean the action is happening right now?

It can, but it doesn't have to! Finnish doesn't have a separate continuous tense like English does. Lapsi syö can translate to either The child eats or The child is eating depending entirely on the context.