Dear Mike,
Thank you very much for inviting me to your birthday party yesterday.
I am afraid I have another appointment at the same time.
Please enjoy yourselves.
Yours Sincerely,
Li Hua
Dear Mike,
I have already received your invitation to your birthday party. And I am very glad togo to your birthday party. But I have to say sorry because I have something to do and can't go to it. I wish you a happy birthday.
Your friend
Li Hua