1)can 用于否定句cannot(help)but表示不能不，只能(but后跟不带to的动词不定式)

　　2)must 表示禁止，一定不要时的否定式为mustn't 当它表示有把握的推断时 意为一定 准是时 它的否定形式为can't

　　3)need doing=need to be done 这个句型表示被动意味

　　4)need not have done sth 表示本来没有必要做某事 (经常考)

　　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 must have been apparent the moment I walked into the room, for the first man I met asked sympathetically: “Are you feeling all right?”

　　(2)can’t/couldn’t 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 couldn’t have received my letter; otherwise she would have replied before now.

　　(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 Power’s Crystal River plant, a potentially serious leakage of radioactive water may have been unknowingly caused by an electrician.

　　几个情态动词常考的句型

　　(1)may/might (just) as well“不妨，最好”，与had better相近。如：Since the flight was cancelled, you might as well go by train.

　　(2)cannot/can’t…too… “越……越好，怎么也不过分”。注意这个句型的变体cannot…over…。如：

　　You cannot be too careful when you drive a car.

　　The final chapter covers organizational change and development. This subject cannot be overemphasized.

　　(3)usedn’t或didn’t use to为used to (do)的否定式。

　　(4)should 除了“应该”一层意思外，大纲还规定要掌握其“竟然”的意思。如：I didn’t expect that he should have behaved like that