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标题: 情态动词的用法归纳(4) [打印本页]

作者: hellokitty    时间: 2012-7-6 08:37
标题: 情态动词的用法归纳(4)
  七、 should, ought to
    1) should, ought to表示“应该”,ought to表示义务或责任,比should语气重。
    1. I should help her because she is in trouble.
    2. You ought to take care of the baby.
   
    2) 表示劝告、建议和命令。should, ought to可通用,但在疑问句中常用should。
    1. You should / ought to go to class right away.
    2. Should I open the window?
    3) 表示推测
    should , ought to (客观推测),  must(主观推测)。
    1.He must be home by now. (断定他已到家)
    2.He ought to/should be home by now.(不太肯定)
    3. This is where the oil must be.(直爽)
    4. This is where the oil ought to/should be.(含蓄)



    八、 情态动词+不定式完成式(have done)
    1) can / could + have done在肯定句中表示“本来可以做而实际上能做某事”,是虚拟语气;在疑问句或否定句中表示对过去行为的怀疑或不肯定, 表示推测。
    1. You could have done better, but you didn’t try your best. (虚拟语气)
    2. He can’t have been to that town.(推测)
    3. Can he have got the book?(推测)

    2) may / might +不定式完成式(have done)
表示对过去行为的推测。不能用于疑问句中,没有虚拟语气的用法。Might所表示的可能性比may小。
    1. He may not have finished the work .
    2. If we had taken the other road, we might have arrived earlier.

    3)must +不定式完成式(have done)
用于肯定句中,表示对过去行为的推测。意为“一定、想必”。其疑问、否定形式用can,can’t代替。参看1) can / could + have done表示推测。
    1. You must have seen the film Titanic.
    2. He must have been to Shanghai.

    4)should +不定式完成式(have done)
    用于肯定句中,表示对过去行为的推测。
    He should have finished the work by now。
    表示“本应该做而实际上没有做某事”,其否定式表示某种行为本不该发生却发生了。可以与ought to +不定式完成式(have done)互换。
    1. You ought to / should have helped him. (but you didn’t.)
    2. She shouldn’t have taken away my measuring tape, for I wanted to use it.

    5) needn’t +不定式完成式(have done)
    表示“本来不必做而实际上做了某事”。
    You needn’t have watered the flowers, for it is going to rain.

    6) will +不定式完成式(have done)
     主要用于第二、三人称,表示对已完成的动作或事态的推测。
     He will have arrived by now.






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