【1】
Dear,
I am writing this letter to express my apology that . I feel terribly sorry about this.
Once again, I am very sorry for any inconvenience caused. I will be really appreciated if you can accept my apologies and understand my situation. Yours Sincerely
亲爱的，(对方姓名)
我写这封信以表达我的道歉 (道歉原因)。
对此我深感抱歉。
我再次对给您造成的任何不便深感歉意，希望您能理解我的处境并接受我的道歉。
谨启
(自己姓名)
【2】
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
【3】
Dear Miss Nancy,
Much to my regret I was unable to keep my promise to attend your birthday party last Saturday, owing to the fact that my little son was suddenly taken ill early that day.
Hoping to see you soon.
Truly yours,
Tom