参考答案

正确答案：
Dear Tom,
　　Thank you so much for inviting me to go with you and see your family at Chengde this weekend. I've heard a lot about your kind and warm-hearted parents from you and hope to see them for a long time. And chengde is a beautiful city I dream to visit.
　　But I have to say sorry that I cannot go. Unfortunately, my uncle is coming to see me this weekend on Saturday and on Sunday I have an English exam. So, I have to stay here.
　　I do hope that we can do it another time. Please send my best wishes to your parents.
　　Thank you again and take care.
　　Yours,
　　Li Yuan