How to be a Good Coach

No, I'm not a coach. I only can teach them the basics... Anyway, if they always make a wrong movement, how to fix it ? I've told them but they still make the same mistakes several times. Second, there's my friend that have play table tennis for a long time and he can hit the ball well. But the problem is, even the ball is good, but all of the movement are wrong, from the start position, to the finish position, and even the basic stance. So how to fix it ? It has been a habbit for him and even I force him to make a good stance, he still can't do it and do the wrong habbits. I afraid that it will disturb his play in rally and can't move. Another bad habbit is that he likes to standing far from the table and hit the ball below the table. It's very fatal and until now he still make the wrong habbits... This make me confuse as my another friends just need several times to correct their movement. Please help me...