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

　　should, ought to

　　1) should, ought to表示“應(yīng)該”，ought to表示義務(wù)或責(zé)任，比should語氣重。

　　1. I should help her because she is in trouble.

　　2. You ought to take care of the baby.

　　2) 表示勸告、建議和命令。should, ought to可通用，但在疑問句中常用should。

　　1. You should / ought to go to class right away.

　　2. Should I open the window?

　　3) 表示推測，should , ought to (客觀推測), must(主觀推測)。

　　1.He must be home by now. (斷定他已到家)

　　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)

　　情態(tài)動詞+不定式完成式(have done)

　　1) can / could + have done在肯定句中表示“本來可以做而實際上能做某事”，是虛擬語氣;在疑問句或否定句中表示對過去行為的懷 疑或不肯定, 表示推測。

　　1. You could have done better, but you didn’t try your best.(虛擬語氣)

　　2. He can’t have been to that town.(推測)

　　3. Can he have got the book?(推測)

　　2) may / might +不定式完成式(have done)表示對過去行為的推測。不能用于疑問句中，沒有虛擬語氣的用法。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，表示對過去行為的推測。意為“一定、想必”。其疑問、否定形式用can，can’t代替。參看1) can / could + have done表示推測。

　　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，表示對過去行為的推測。

　　He should have finished the work by now。

　　表示“本應(yīng)該做而實際上沒有做某事”，其否定式表示某種行為本不該發(fā)生卻發(fā)生了�？梢耘cought to +不定式完成式(have don e)互換。

　　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“本來不必做而實際上做了某事”。

　　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、三人稱，表示對已完成的動作或事態(tài)的推測。

　　He will have arrived by now.

　　情態(tài)動詞表示猜測

　　肯定：must>should>could>may>might

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

　　表示要求，命令時，語氣由 should(應(yīng)該)、had better最好)、must(必須)漸強。

　　必須掌握情態(tài)動詞表示推測語氣時的反意疑問句。

　　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? (多了過去時的時間狀語)

　　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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