Dear Yangling:
Congratulations on your coming birthday.
Thanks for your invitation, but I am sorry that I am afraid I am not able to att your birthday party.
You know, there is an important English test in our school on the same day. I will have to att 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