Dear Mr Smith,
Thank you very much for inviting me to Beijing and helping me with English To be honest,I am longing to visit you because l have not seen you for a long time.However,the current situation does not allow me to do this.
My grandmother,who lives in the country,is very lonely and expects me to spend the coming vacation together with her.She misses me so much that I have no choice but to obey her.I feel very sorry to disappoint you.I hope to be able to visit you and Tom at the end of the year.I do appreciate your invitation.Meanwhile,I wish you and Tom all the best.
Yours sincerely,
Li Hui
参考译文：
史密斯先生：
我很高兴您能够邀请我去北京玩并且在英语上帮助我。说实话，我很期待能去拜访您，因为我好久没见到您了。 然而，目前的情况不允许我这样做。
我的祖母住在乡下，她非常孤独，希望能和我度过暑假。她很想念我，我不想违背她的意愿。 不好意思让您失望了，我希望能够在今年年底再去拜访您与汤姆。 我非常感谢您的邀请。祝您和汤姆一切顺利。
真诚的，
李辉