【正确答案】Dear Mr. Smith,
   We feel greatly honored to invite you, Mr. Smith, to give a talk on "American History" to students of the English Department on Saturday, April 19 (from 2 p.m. to4 p. m. ). We should be very grateful if you can come that day. If the subject does not suit you, any other similar topic would also be welcome.
   We have already had several very interesting talks delivered by some professors from various countries this semester. We are looking forward to the opportunity to benefit from your experience and wisdom.

【答案解析】邀请信一般适合用于某些普通场合，并且，邀请者和受邀请者都比较了解彼此。邀请信的特点是简洁而充满热情。
   内容：
   1．应告知受邀请者其受邀参加的活动及受邀原因。
   2．应告知受邀请者诸如时间、地点、参与者、你的准备情况以及服饰风格等具体情况。
   注意事项：
   1．请柬应该用第三人称写，而邀请信用第一人称写。
   2．如果你希望收到被邀请者的答复，你得在请柬上注明“R. S. V. P. ”或“r. s. v. P. ”，意思是“请答复”。必要的时候可以写上你的电话号码和地址。
   3．请柬应该居中写。