考生必備，建議收藏。
一、can 與 be able to do 的區別
1）時態： can 只有 現在時 和 過去（could)
be able to do 有多種時態。
在 將來時 ，完成時 以及 非謂語動詞中應該用 be able to do.
e.g.——So far, doctors have been able to do very little to treat this kind of disease.
2) can 表一貫的能力，
be able to 表示客觀能力 和通過努力可以達到的能力
e.g.—— I can't swim.
But I am sure I will be able to swim through more practicing.
can't (help) but do 不得不做，只好做
can't help dong 情不自禁
二、must 與 have to
must 1）表必須 ， 其否定形式通常表示禁止。
e.g.—— You must do your homework now.
You mustn't smoke here. 這裡禁止吸菸。
2） 表推測， 意思是“一定”，一般用於肯定句
e.g.——He must be on the playground.
- must 用於一般問句中，肯定回答用 must ，否定式用 needn't 或 don'thave to ，做“不必”；mustn't 表示“禁止，不允許”
- 表示“必須”這個意思時， must 和 have to 稍有區別 。
must 著重說明主觀看法，
have to 強調客觀需要 。 另外，have to 能用於更多時態 。
e.g.——I don't like TV set. We must buy a new one.
There was no more bus .They had to walk home.
三、 may / might have done
表示對過去已經發生的行為的推測， 意為“ 也許 / 或許已經（沒有）”
e.g.——It's too late. I think he may have gone to bed.
四、should / ought to have done
用於肯定句時，表示“ 本應該做某事，而實際上未做 ”
用於否定句時，表示“ 不應該做的事情反而做了
e.g.—— You should have come to the meeting earlier .
You ought to have done this exercise more carefully.
五、 needn't have done
表示“ 做了本來不必去做的事情 ”
e.g.—— You needn't have taken a taxi here ,for it was very near to my house.
希望大家的英語成績都能有所提升！
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