How to study well English grammar?
My advice is to learn English grammar through books. You should read children's books and English books. If not, then you should buy bilingual funny books to read both entertainment and remember the grammar structure used inside. From there, learn, take notes to apply when needed.
And, what is your advice?
I agree with this. Learning through reading books and novels are a good way to pick up good English in regards to grammar. I also suggest regularly using the knowledge learned thru this. This improves muscle memory by making you get used to actually using the language. In the long run, you slowly become comfortable and can easily carry a conversation. Additionally, the "reading + using" technique will also improve comprehension.