[解析]
Dear John,
I was excited to hear that you would come to China next month.Now let me tell you something different you may meet here.(点明写信目的)To begin with,the weather here is totally different from that in your place.It hardly ever rains but whenever it does,it rains heavily.【高分句型一】So you'd better bring a raincoat and overcoat.Another big difference is food.The Chinese food is quite different from yours.You have to try to get used to it,and learn how to use chopsticks.【高分句型二】I'm sure you can do it.(阐述他可能遇到的一些不同于美国的情况)
Finally please bring me a big American map if possible.Thank you very much.I am looking forward to meeting you.(请他带一张美国地图,希望早日能见面.)
Yours
Zhang Nan