When you learn a foreign language, you naturally need to learn the grammar rules and a lot of words. But if you understand the rules, that does not mean that you can already apply the rules.
One can compare this with the learning of a musical instrument. If you know what to do, that is not enough. You need to practice it again and again.
I developed the exercises on the following pages, so that my students have enough exercises for learning the German language. I hope the exercises help you too.