考研的英语语法速成：情态动词现在完成式用法

　　情态动词现在完成式主要有两个功能：表示已经发生的情况和表示虚拟语气。在这两个方面must/mustncan/cannneed/neednmay/maynmight/mightnshould/shouldnought等情态动词+完成式表示的意思是有一定区别的

　　1.表示已经发生的情况。

　　1)must have+过去分词，表示对已发生情况的肯定推测，译为(昨天)一定。如：

　　My pain apparent the moment I walked into the room, for the first man I met asked sympathetically: Are you feeling all right?

　　[A] must be [B] had been [C] must have been [D] had to be

　　(答案为C)

　　2)cant / couldnt have+过去分词，表示对已发生情况的否定推测，译为(昨天)一定没。如：

　　Mary my letter; otherwise she would have replied before now.

　　[A] couldnt have received [B] ought to have received

　　[C] has received [D] shouldnt have received

　　(答案为A)

　　3)may / might have +过去分词，表示对已发生的事情做不肯定、可能性很小的推测，或事实上根本没发生，译为也许。如：

　　At Florida Powers Crystal River plant, a potentially serious leakage of radioactive water may have been unknowingly caused by an electrician.

　　2.表示虚拟语气。

　　1) neednt have + 过去分词，表示做了不必做的事，相当于didnt need to do,译为其实没必要。如：