Dear Yangling:Congratulations on your coming birthday. Thanks for your invitation, but I am sorry that I am afraid I am not able to attend your birthday party. You know, there is an important English test in our school on the same day. I will have to attend the test . I am terribly sorry for that. Anyway, I hope everything goes well in your birthday party. Remember to show me the pictures taken in the party.Best regards! Yours ever Susan