Dear Mr. Black，
I’m very thank to you for inviting me to stay with you in the come summer vacation. I have been dreamed of seeing you again so that I can get help from you in my English learning. And I’m afraid I cannot go to your place in this summer. I’ve promised my grandmother, she lives in the country, to spend this summer with her. She misses me too much that I find difficult to refuse her request. I’m busy preparing for my examinations in present. I planned to visit you in winter if it’s convenient to you.
Best wishes.
Yours truly,
Li Ying