Not much, in fact. There are some not easy aspects in the English Grammar, but, mostly, it isn't that hard to learn, to be honest.

Is English grammar difficult to learn? ("Grammar" is never plural.) Are English grammar rules difficult to learn? No, they're not hard at all. If the concepts are explained clearly, it's all really very simple. Of course, there is a certain amount of learning, practice and remembering. That'd be the hard slog.

Yes. Correction: Is English grammar difficult to learn?

A good teacher will help you to find the logic in grammar, and that makes it much easier to understand.