导学案4情态动词（ii）
一、ought to 应该，应当 其否定 “ought not to” 或 “oughtn’t to”
(1) 用于第一人称，表有责任或有必要做某事。
we ought to be more careful with our homework.
(2) 用于二、三人称，表建议或劝告。
you ought to follow your teacher’s advice.
she ought not to go alone.
(3) ought to+have done: 本该……
i’m sorry. i ought to have told you this morning.对不起，我本该今天早上告诉你的。
二、have to / don’t have to / mustn’t
(1) have to = have got to 必须，不得不 （有人称和数、时态的变化）
he has to work on the farm all day . 他不得不在农场工作一整天。
（2）don’t have to 不必……
you don’t have to buy a computer.
(3) mustn’t 禁止，不许
you mustn’t sleep in class!
三、need 需要，必要（既可以作情态动词，也可以作实义动词，但用法不同）
（1）作为情态动词，need 通常用于否定句和疑问句
①you needn’t buy the book..
②——need he do his homework first?
——yes ,he must! (no ,he needn’t)
(2) need 作为实义动词时，通常用法是：
sb/sth need to do (表主动) we need to tell him the truth.
sb/sth need doing /to be done（表被动） my car needs repairing./my car needs to be repaird
sb need sth 某人需要…… mary needs your help.
三、情态动词+have done
1. must have done 推测过去“肯定……”
he must have played computer games last night.
2 may / might have done 推测过去“也许……”
he might have known the answer to the question.
3.(1)could have done 用于疑问句时，表示 推测过去“可能……？”
could he have finished the task? 他可能把任务完成了吗？
（2）could have done 用于肯定句时， 表“能够做而没有做”
i could have finished the work on my own. 我本能够独自完成工作.
4. (1)should have done 本应该做而没有做
you should have studied hard.
(2) shouldn’t have done 本不该做而做了
you shouldn’t have told lies.
5. ought to have done: 本该做而没做
6. needn’t have done 本不必…… 本没必要……
you needn’t have told me the news.
