情態動詞ought to的用法總結
一、Ought to表示應該。如：
1.You ought to take care of him.
你應該照顧他。
2.You ought to scrap the old bike and buy a new one.
你應該扔掉這輛舊自行車，買一輛新的。
3.You ought to have a haircut.
你該去剪髮了。
二、表示推測。注意與must表示推測時的區別：
He must be at home by now. （斷定他已到家）
He ought to be home by now. （不十分肯定）
This is where the oil must be. （比較直率）
This is where the oil ought to be. （比較含蓄）
三、“ought to + have + 過去分詞”表示過去應做某事而實際未做。如：
You ought to have asked him (but you didn't).
這時，ought to和should可以互相換用。
注意：在美國英語中，ought to用於否定句和疑問句時，to可以省略。如：
Ought you smoke so much?
You oughtn't smoke so much.
四、ought和should的區別：
ought語氣略強，should較常用；ought在美國英語中用的很少，而should卻相當常用；
ought屬正式用語。由於ought to 沒有過去式，所以在直接引語變間接引語的過程中，就不再變化.例如：
1.She failed one of the exam papers, but we ought to make allowance for the fact that she was ill.
她有一門不及格，但我們必須考慮到她當時有病。
2.Regularity ought to be observed, as regularity is very conducive to health.
生活應遵守規律，因爲規律對健康有益。
3.China ought to make a greater contribution to humanity.
中國應當對於人類有較大的貢獻。
4.He is a very good tennis player; you ought to see him in action.
他是個很好的網球運動員，你應當去看看他打球。
5.I think there's a train at 8.20 but you ought to make certain.
我想8點20分有一班火車，不過你應該打聽清楚。
6.I think you ought to go to/to go and see the doctor.
我認爲你應當去找醫生看看病。
