【正确答案】Dear Prof. Smith,
On behalf of our university, we would like to extend to you an invitation to be our guest speaker at the annual academic conference.
The conference will be held at the meeting room at nine o'clock a. m. on December 10^th, 2012. As you know, the department is interested in the 20th century English literature. Since you are familiar with this field, we know your views will be extremely interesting to us. Please confirm that you will be able to attend by advising us of your time, so that we may complete our agenda and arrange for you to be met.
We would appreciate it very much if you could accept our invitation.