Dear Tom,
Thank you very much for inviting me to your birthday party. I'd love to go but I can't.
You know, my parents are currently on a business trip, and they will not return until next month. My grandmother is old and in poor health, so I have to stay home all the time to look after her. And I'm also busy recently preparing for the upcoming English test.
So I'm really sorry that I can't make it this time. Thank you once again and wish you a very happy birthday and a wonderful birthday party.
Yours sincerely,
Li Ping