【正确答案】Dear Tom,
I am very much delighted and honored to have received the invitation to your birthday party. It is really nice of you to think of me for such an important occasion. However, it is really depressing that I may not be able to attend it.
I will be coincidentally unavailable on that day. One of my overseas friends will come to visit me and the problem is that he will stay here for on]y one day. Though I myself like to attend the party very much, I am also expected to stay with him. Thinking that it is easier for us to see each other in future, I hope you will kindly accept my choice.
I am really sorry that I can not enjoy the party. I would like to, however, say "Happy Birthday" in advance and to express my best regards for your brighter future life.
　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　 Cordially,
　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　 Li Ming