It's all very simple, Davood. We use GO to describe movement AWAY FROM THE PLACE or position where you are. We use COME to describe movement TO THE PLACE where you are. The same rule applies with GO BACK and COME BACK. From your friend's point of you 'you go back'. From your own point of view 'you come back'. But you can use RETURN for both GO BACK and COME BACK. Imagine this situation. Your friend Jack living in Spain is returning to London where he was born and his family lives. He would say to you, "Tomorrow I'm coming back to London." So Jack left for London the next day. After a while, another friend of yours is wondering where Jack is. In that case, you can tell him, "He's gone back to London." Hope this helps! ;)