I read some things in English, and I can understand but when it comes to writing because I can not? --> If I can read in English, why is it so difficult to write in English?
It's similar to learning how to play tennis. You can read a lot of books about how to play the game, but you actually have to pick up a tennis racket and hit a ball to truly understand it.
It's the same with writing. You need to exercise your writing muscle. :)