Dear Mr Li,
I'm very excited to write to you.You have taught me English for three years.And in the three years,you were serious but friendly to us.With the help of you,my English has become better and better.Although I may make some mistakes in a test,you never blame me.What you did is encouraging me to believe myself.It made me have enough brave to continue my English study.So I really want to say" Thank you "to you.I wish you will be happy and healthy in the future.