Dear Mr. Smith:
It's been three years, isn't it? I can still remember the first day in your class learning English. It was hard and challenging subject to learn in school. I struggled through the class and I manage to get decent grades in the end and learn a good amount of English. I will continue to persue in learning English and get better and better each day. I have such a great time learning English in your class and happy to have you as my teacher.
Sincerely,
XXX