Dear teacher,
I'm XXX, one of your students in XXX class. In my point of view, your class is fantastic. I'm really glad that I have the chance to be your student.
Today I reviewed the information that I learned in your class. However, I found something I don't understand. For instance, you said XXX, but I think XXX is better.
The document of XXX is attached.
Look forward to getting your answer.
Sincerely,
XXX