Dear Mr Smith,
I’m terribly sorry to tell you that I can’t join in the outing that you will organize(一般将来时) on Sunday morning.I broke my foot yesterday afternoon,when I was playing basketball with my friends.I will have to stay in bed for several days till it recovers.(将来完成时),It is a great pity for me to miss such a good chance to spend time with you. I (最好不用would,虚拟语气表示不真实的,如果你这里用虚拟语气,别人觉得你很假)will be very happy if you could give me your kind understanding. And I hope you will have a good（最好用happy吧,） time.
句子结尾用 your sincerely
li hua(随便写的名字)
这就是高考作文的基本套路了,还有注意断句,标点,不要一个句子写到尾,随时要断句.不然阅卷老师费解,没什么大的语法错误,得18分应该没问题,主要是你结尾没落款写信人的名字.