Dear Mary,
Thank you very much for inviting me to your birthday party.I'd love to go but I can't.My parents are out on business.They are coming back next month.And now I have to look after my grandma because she is ill in bed.And I also need to study for my coming English exam.I'm very sorry I can't go to your party.Thank you for asking me.I hope you can have a good time.
Meixian