语法复习十一：助动词与情态动词_高考语法

语法复习十一：助动词与情态动词

　　4、can, could, may, must后接完成式的用法

　　(1)can, could后接完成式的用法：①在否定句或疑问句中表示对过去发生过的事情的怀疑或不肯定的态度，Could he have said so? ②在肯定句中，可以表示过去可能做到而实际并没做到的事情，有劝告或责备的语气。如：---- When did you answer her letter? ---- Only yesterday. ---- Its too late. You could have answered it earlier, I am sure. (2)may, might后接完成式的用法 ①表示对过去某事的推测，认为某一件事情在过去可能发生了。如果使用might，语气就比较婉转或更加不肯定，如：Mary might have learned some Chinese before. ②可以表示过去本来可以做到而实际没有做到的事情，有劝告或责备的语气，如：You didnt do the work well that day. You might have done it better. (3)must后接完成式的用法：表示对过去某事的推测，认为某事在过去一定做到了，如：Liu Dong isnt in the classroom. He must have gone to the library.

　　5、have to 的基本用法：have to和must的意义相近，只是 must侧重表示说话人的主观看法，而have to 则表示客观需要，如：I must study hard. I had to give it up because of illness.

　　6、ought to的基本用法

　　(l)表示根据某种义务或必要应当做某事，语气比should强，例如：Everyone ought to obey the traffic regulations. (2)表示推测，注意与must表示推测时的区别：He must be home by now .(断定他已到家)，He ought to be home by now .(不十分肯定)，This is where the oil must be.(比较直率) ，This is where the oil ought to be. (比较含蓄) ；(3)ought + have+ 过去分词表示过去应做某事而实际未做。例如：You ought to have helped him. (but you didnt) 这时，ought与 should可以互相换用。注意，在美国英语中ought to 用于否定和疑问句时to可以省略。例如：Ought you smoke so much? You oughtnt smoke so much.