Dear Felix
Thank you very much for the extra help you gave me so I could pass my statistics class. I know how busy you are, so I really appreciated the time you spent with me. This class was the only thing standing between me and my degree, and, as you know, I had a very hard time understanding and remembering the concepts. Thanks to you I will be graduating in June. You are the kind of professor that makes this university a great place to learn.
Regards
Lydia