dear tom,
i'm very happy that you invite me to your birthday party this week, but i am very sorry that i cannot make it. my parents went out for work and can only return next month, but my grandmother is sick in bed so i must take care of her. i also have piano lesson this week and lots of homework to do. besides, our english exam is coming and i must study for it. therefore i will not be able to go for your party, i am terribly sorry. happy birthday!
regards
li ping
该大写的地方麻烦你自己改吧~嘿嘿