Education English | Grammar : Use of Modals " Must and Ought to " | 1. Modal auxiliary must used for :
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.
Negative form is oughtn't (in the sense should not be done)
musn't (do not , ban)