Dear Tom,
I am Lisa.Thanks for your invitation.I am glad to go but I can not just for some reasons.
On the one hand,my parents have worked out untill next week.During this period of time,I have to stay at home to look after my grandmother who has a illness .
On the other hand,the English exam is approaching,I must review the kowledege that I learned in order to pass the exam.
So you can see that I am really very busy at present,Although I can not go to you birthday party this time,I wish you a happy birthday.
Yours,
Lisa.
原创哦,呵呵,希望对你有所帮助~