Dear Ted,
Thanks for inviting me to climb the mountains this weekend.I'd love to go with you,but I'm afraid I can't go there because I have something important to do on that day,it's my grandmother's birthday.On Saturday morning I'll go to the shops,then I'll go to my grandma's,before dinner we'll stay there with her.Besides,I'll have a piano lesson in the evening.How busy I'll be on that day!Maybe we can go mountain clinbing together next time.Thank you anyway.
Yours,
Li Ming