Dear ×××,
I want to thank you dearly for all that you have done for me and especially for the exersises you help me last week.
I understand how much of your valuable time you spent on me and my problems. And knowing how busy you are, I truly appreciate your assistance in this matter.
Maybe some time in the future I will be able to repay you to some degree for all that you have done for me.
Yours,×××