English Grammar : Use of Modals " Must and Ought to "

Learn Grammar | English Grammar : Use of Modals " Must and Ought to " | 1. Modal auxiliary must used for :

a. Stating the necessity
You must go before raining.
We had to see the doctor two hours ago.
Must not have a past tense, past tense of must we use "had to" coming from "have to".
Differences use "must " and "have to":
Must: When must come from outside the speaker
Have to: When must come from the conscience of the speaker.

b. Stating the meaning of certainty.
The key must be in her bag.
Niken must be studying now.
I believe that she must know about the matter.

2. Modal auxiliary ought to used for :
Meaning should be stated, but not the same as "must" because they have to be here to do.
He must do the work
He ought to do the work.

Negative form is oughtn't (in the sense should not be done)
musn't (do not , ban)
You oughtn't (to) go to the movie
Past form of ought to is : ought to have
He ought to have gone to the movie yesterday.
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