I am writing this letter just want to say thanks to you. I am going to graduate in two weeks. You have been my class adviser for three years. During these three years you have spent lots of energy on us. You are not only teaching knowledge in the class, but also care our life.

Dear Mr. Lai, 

尊敬的赖老师： 
I am writing this letter just want to say thanks to you. I am going to graduate in two weeks. You have been my class adviser for three years. During these three years you have spent lots of energy on us. You are not only teaching knowledge in the class, but also care our life. Sometimes we are afraid of you, because of your strict requirement for our study. However, you are so helpful in the daily life. No matter what difficulties we are in, you will help if you can. So, I also want to say sorry to you for making so much trouble. But you bear all my trouble. I don’t know what kind of class adviser I will meet in the future study, but I will remember and appreciate you forever. I want to say thank you again. 
我写这封信只是想跟你说声谢谢。还有两周我就要毕业了。你做了我三年的班主任。在这三年你花了那么多精力在我们身上。你不仅仅在课堂上传授我们知识，而且在生活上也很关心我们。不过有时候我们有点怕你，因为你对我们学习的严格要求。不过，在日常生活中你是那么的乐于助人。无论我们遇到什么样的困难，只要能够帮得上忙你都会帮我们。所以，我还想对你说声对不起，给你带来那么多的麻烦。但是你都一一包容了。我不知道我会在今后的学习中遇到什么样的班主任，但是我会一直记得你、感激你的。我想再次说声谢谢。
Regarded
Student 
学生