The general design of Duolingo is to introduce grammatical concepts to you and have you learn by deduction then have them explained when you get to a skill dedicated to that topic. Most languages on here are set up this way. It's pretty difficult for them to teach all the grammar before you are introduced to it, and you are much more likely to remember a grammar rule when you've seen it in practice THEN learn the rule, because it explains something you've already noticed as peculiar or confusing. I find reading the grammar chapters after trying at least one lesson in the skill makes it so that I actually understand what I'm reading (instead of just skimming the information) and I am able to connect it with things I have already seen. This is part of why this is seen here but explained later on. In addition, anything you do not understand you can ask here (like you did) and receive explanations from the community. Asking questions and figuring concepts out is more useful than a wall of text about something.

ETA: I'm laughing at myself bc I wrote a wall of text talking about how reading a wall of text means you're less likely to learn something from it. the irony!

I agree with you katsiano, I understand the grammar rules a lot better after I've done a lesson. I typically do the lesson, review the notes, do some other lessons then head back to the old lesson. I actually just did this with colors and learned a lot from it.