英语情态动词用法整理，重难点全覆盖(4)

　　In those days, whenever I had difficulties, I would go to Mr. Chen for help.

　　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)

　　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 don e)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.

　　情态动词表示猜测

　　肯定：must>should>could>may>might

　　否定：can’t>shouldn’t>couldn’t>may not>

　　表示要求，命令时，语气由 should(应该)、had better最好)、must(必须)渐强。

　　必须掌握情态动词表示推测语气时的反意疑问句。

　　例如：

　　The road is wet. It must have rained, hasn t it?

　　The road is wet. It must have rained last night, didn t it? (多了过去时的时间状语)

　　I met him on his way home. He can’t be in the library now, isn t he?

　　The coat is too close to the fire. It might get burnt, doesn t it?

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