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...

I see... Oh yeah, one more problem. When they can perform their basic stroke correctly, then after serveral days, they made another mistake that the stroke movement is wrong and they can't do it just like before. Do you know what's the problem ? It's very sucks when they can do the right stroke continuously then make many faults again... How to fix it ?

Ok, thank you for the advice I'll always remember to be patience :D

Hi PingSkills. There's another problem on teaching the stroke. That's about their consistency. They like to hit the ball fast and sometimes it make the ball spinned but the ball is not consistence. So how to fix it ? I've told them and give them examples but they still make that mistake. I know patience is the key but what should I do to make their movement slow but have a lot of consistency so the stroke will be correct. Should I use multi-ball or what ? And how ? Please for the answer. Thank you very much