Ought to 係用來表示勸告、建議的「應該」。它亦可用來表示假設、預期以及高度可能性 (或然率) 的「應該」，這通常含有某人該得或應得某物或某事的概念。Ought to 的否定式為 “Ought not to” (否定式的 to 可以省略)，係用來勸告 (某人) 不要做某事，不過美國人比較喜歡使用較不正式的 should not 或 had better not。Ought not 可以縮略為 oughtn’t。
Ought to 的過去式是 ought to have + P.P.，主要在表達與過去事實相反的情況，如 You ought to have/should have helped him. (你本來應該幫助他的 － 但實際上卻沒有)。過去式的否定也是在 ought 的後面加上 not，如 You ought not (to) have/should not have helped him. (你本來不該幫助他的 － 但實際上卻伸出了援手)；He ought not (to) have done so. (他本來不該這麼做的 － 但實際上卻做了)。
例句：
- You ought to give up smoking. (你應該戒煙) － 勸告、忠告
- We ought to go swimming on weekends. (我們應該每週末游泳) － 建議
- Their meeting ought to be over now. (他們的會議現在該結束了) － 預期、可能性 (我預計會議應該結束了)
- Tony ought to get the promotion. (東尼該獲得擢升) － 預期、可能性 (預料東尼會獲得擢升，而晉升是他應得的)
- Sam ought not (to) eat so much. (山姆不該吃這麼多) － 勸告不要做某事
Ought to 的現在、過去和未來時態用法
大多數語氣助動詞的過去和未來時態都相當不規則。下表為 ought to 在不同上下文中的用法說明。
| Ought to 的用法 | 肯定句 | 你也可以用 |
| 勸告、建議 | 現在：Susan ought to exercise more. (蘇珊應該多運動) 過去：Susan ought to have exercised more so she would be better prepared for the marathon. (蘇珊應該多運動，這樣她才能為這項馬拉松做較佳的準備) 未來：Susan ought to go jogging with us tonight. (蘇珊今晚應該跟我們去慢跑) | should |
| 假設、預期、可能性 | 現在：Ivan ought to be home at 7 o'clock today. (艾凡今天七點鐘應該回家了) 過去：Mary ought to have received my package by now. (瑪麗現在應該已經收到我的包裹了) 未來：Mary ought to receive my package tonight. (瑪麗今晚應該會收到我的包裹) | should |
| Ought to 的用法 | 否定句 | 你也可以用 |
| 勸告、建議 | 現在：Susan ought not (to) exercise too much. It might cause injury. (蘇珊不該做太多運動。那可能導致受傷) 過去：Susan ought not (to) have run marathon. She wasn't in good shape. (蘇珊不該跑馬拉松。她的身體狀況不佳) 未來：Susan ought not (to) stay at home in front of the TV. She should go jogging with us tonight. (蘇珊不該留在家裡看電視。她今晚應該跟我們去慢跑) | should |
| 假設、預期、可能性 | Ought not 主要用來勸告 (某人) 不要做某事。(請見上面的例句) | should |
注意：“Ought to” 的否定式為 “Ought not to”，其中 to 大多省略。Ought not 及其縮略形式 oughtn’t 主要為英式英語用法，在英國用得非常頻繁，但在美式英語中卻非常少用，美國人偏愛 should not。
例句：
- You ought not (to) drink so much. (你不該喝這麼多)
- You oughtn’t (to) go out with Amy because she isn’t a good girl. (你不該和艾美交往，因為她不是好女孩)
- Gary ought not (to) have made so much noise last night. (蓋瑞昨晚不該那麼吵)
在 ought not to 的用法中會有「假設、預期和可能性」的情況嗎？
e.g.
1. A: What Tony wins is just a fluke.
B: Yep! He ought not (to) win the prize.
2. Mary ought not to have had an abortion last summer.
這兩句都有這樣的含意！
Admin, thanks for your sincere reply. Yet you might misunderstand what I mean. As a matter of fact, What I’m wondering is why the author left blank in the place for the negative usage of “ought to” under the premise for prediction. Is that because “ought to” don’t have this kind of statement? So, I made the two words to see if it’s correct to utilize them under these kinds of circumstances.