Learn Finnish: Kun syö kohtuudella, hyvinvointi parantuu. - When one eats in moderation, wellbeing improves.

This is the generic zero-person construction. In Finnish, you can make a general statement that applies to anyone by using a verb in the third-person singular (like syö) without a subject. It translates to "when one eats" or "when you eat" in a general sense.

What is the base form of kohtuudella?

The base word is the noun kohtuus, which means moderation. Here it is in the adessive case (-lla), which expresses how something is done, literally meaning "with moderation." Notice how the word stem changes from -uus to -uude- before the case ending is added.

Why use the verb parantuu instead of parantaa?

Parantua is an intransitive verb, meaning something improves or gets better on its own. Parantaa is a transitive verb, meaning someone actively improves or cures something else. Since wellbeing is improving on its own here, parantuu is the correct choice.

Is it common to use the word "you" for general statements like this in everyday speech?

In spoken or casual Finnish, it is very common to use the "you" form (like kun sä syöt) for generic statements, similar to English. However, the zero-person construction without a subject is the traditional and grammatically correct way to express this in standard Finnish.