【正确答案】Dear Jack,
Much to my regret that I am unable to attend your birthday party next Tuesday evening．That is owing to the fact that our physiological teacher has assigned us quite a few experiments and required us to hand in our reports of the results before next Thursday, and it happens that your birthday party is going to be held when I will be fully occupied by the assignments.
So I am terribly sorry that I cannot go to celebrate your birthday personally and I really regret that I would miss the perfect chance of enjoying myself with so many friends. But I'd like to wish you a happy birthday here and ask the postman to send you in advance a gift which is a small token of my best wishes for you. Besides please give my regards to our friends when you meet them at the party.