Just learning roller skating, I often fall backwards. What should I do?
You don't have to study according to that standard. Learning roller skating like walking is the best way.
First of all, you should "stand upside down", learn to stand first, and then try to move forward slowly after standing firm (bend your knees and focus on the front, but not too far).
Second, find a sense of balance when sliding. It's actually quite simple. Just keep standing when you skate. The key is to keep your feet parallel. The center of gravity should be between your legs.
Third, reducing the frequency of stepping is to lengthen the time of each step. Exercise the balance of one foot in order to lay the foundation for fast sliding.
You said it was easy to lean back because you didn't practice your standing posture well. Slide backwards because the feet are not parallel and the center of gravity is not in front.
Learn roller skating, don't be impatient.