Sorry, I can't type chinese now. I can only reply in english.
Yes. I agreed that coach or stringer should get paid for what they contributed. My question is not focus on demo lession. I just want to see is there any way or guideline to help customer to select their coach ? Especially, when the student is new on Tennis. For me, I will observe few coaching lession for him with other student first. I think I have ability to judge the coach is good or not. But for tennis beginner, I think they will have difficulty. Of course, you can select the coach based on friend's recommendation or you can join the coaching lession which is organized by a reliable organization / website.
I agree that there is a lot of coach in HK, But not so many of them is good or qualified.