开头部分：

How nice to hear from you again.

Let me tell you something about the activity.

I’m glad to have received your letter of Apr. 9th.

I’m pleased to hear that you’re coming to China for a visit.

I’m writing to thank you for your help during my stay in America.

结尾部分：

With best wishes.

I’m looking forward to your reply.

I’d appreciate it if you could reply earlier.