情态动词的用法归纳(4)

hellokitty

  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.