- You must have known it.
- He must go.
Ought was originally the past time form of owe, hence means to be indebted to, to owe. It conveys the idea of a moral obligation, as,
- You ought to help the cause.
- You ought to understand.
179. Ought is always used with the infinitive, and the same form is used to express both the present and the past time. The difference in time is expressed by a change in the infinitive instead of a change in the form of the helping verb. With may and might and can and could, present and past time are expressed by a change in the form of the helping verb. With the helping verb ought, the difference in time is expressed in the infinitive. For example:
- He ought to pay us our wages.
This means, He owes it to us to pay us our wages now.
- He ought to have paid us our wages.
This means, He owed it to us to pay us our wages some time in the past.
180. The present infinitive is used with the helping verb ought to express present time and the perfect infinitive is used with ought to express past time.