2、Must do sth：表示对现在内容的肯定性的推测。

　　二、情态动词用于对过去内容推荐的常见句型：

　　1、can/may have done sth：表示对过去内容的可能性;

　　2、must have done sth：表示对过去内容的肯定性的推测。

　　例：1、Mr Green must have failed to receive my letter, otherwise he would have replied.

　　2、I believe he must have had an accident, otherwise he would have arrived on time.