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：

　　You neednt have come over yourself.

　　As it turned out to be a small house party, we so formally.

　　[A] neednt dress up [B]did not need have dressed up

　　[C] did not need dress up [D] neednt have dressed up

　　2)should have +过去分词，表示应该做某事但实际上未做，译为本应该should not + have过去分词表示本不应该做某事但实际上做了，译为本不应该。如：

　　I regret having left the work unfinished; I should have planned everything ahead carefully.

　　我本来应该事先认真地把每件事情规划的很好，但实际上作者还是没有规划好，以至工作没有完成。

　　3) ought to have +过去分词，表示动作按理该发生了，但实际上未发生，译为该，与should 的完成式含义类似。如：

　　The porter ought to have called the fire-brigade as soon as he saw the fire in the stock, which went up in smoke .

　　4) could have +过去分词，表示过去本来可以做但却未做，译为完全可以。这点与ought/should/ have +过去分词用法相似。如：

　　What you said is right, but you could have phrased it more tactfully.

　　5) may/ might have +过去分词，表示过去可以做但实际未做，译为(那样)也许会。如：

　　It might have been better to include more punchy statistics and photos of equipment in the introduction to further assist first-time office automation managers.