Dear xx,
Thank you very much for inviting me to your party.I am very sorry,but I can’t come.My mother is ill.The doctor has told her that she should stay in bed for several days.I’ll take care of my mother.Thank you very much for sending me such a beautiful postcard.I like it very much.I am going to put it on my desk.
Could I hear from you soon?
Give my best wishes to your family.
Yoursxx,