假如你叫Liu Mei，你的好朋友Mary邀请你去参加她的生日晚会。可是你的妈妈病了，她要去看医生。你必须在家照顾弟弟。英语要考试了，晚上你要在家学习。因此，你太忙，不能去。请你给她写一封信，告诉她你为什么不能去，并祝她生日快乐。（开头与结尾已给出）
Dear Mary,
Thank you for _______________________________________________________________________________
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Write soon．
Yours，
Liu Mei
(Thank you for) inviting me to your birthday party. I really want to go, but mother is ill. She is going to go to see the doctor. I have to take care of my little brother. And the English exam is coming, I should study English at home in the evening. So I am too busy to join your birthday party. Happy birthday to you!
首先谢谢Mary的邀请，然后告诉朋友不能去的原因，使用should，have to。时态上用了一般现在时，一般将来时。不得不留在家里，最后告诉结果，祝朋友生日快乐。