ought

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ought常接不定式符號to,用作情態(tài)動詞表示責(zé)任或義務(wù)時作“應(yīng)該”解,表示勸告或推薦時作“應(yīng)當(dāng)”解,表示推論時作“可能會”解,但都不及must語氣強烈。其否定形式為ought not to,也可縮寫成oughtn't to。
在美式英語中, ought to在否定句或疑問句中可省去to。
在間接引語中,表示過去時ought to形式不變; ought to與完成時連用時多數(shù)情況下表示與過去事實相反的情況,含意為“更早的時間應(yīng)該如何”; ought to還可表示將來的情況,但前面不能加will。
ought to表示推論時在大多數(shù)情況下沒有must肯定,但某些場合也含有很大的可能性。

You use ought to to mean that it is morally right to do a particular thing or that it is morally right for a particular situation to exist, especially when giving or asking for advice or opinions.
You use ought to when saying that you think it is a good idea and important for you or someone else to do a particular thing, especially when giving or asking for advice or opinions.
You use ought to to indicate that you expect something to be true or to happen.
You use ought to to indicate that you think that something should be the case, but might not be.
You use ought to to indicate that you think that something has happened because of what you know about the situation, but you are not certain.
You use ought to have with a past participle to indicate that something was expected to happen or be the case, but it did not happen or was not the case.
You use ought to have with a past participle to indicate that although it was best or correct for someone to do something in the past, they did not actually do it.
You use ought to when politely telling someone that you must do something, for example, that you must leave.