Rule 1. ‘Must' as Modal is used to express a strong sense of duty.
Rule 2. ‘Must' as a Modal is used to express a strong moral obligation.
Rule 3. ‘Must' as a Modal is used to express strong determination.
Rule 4. ‘Must' as a Modal is used to express certainty or strong likelihood.
Rule 5. ‘Must' as a Modal is used to express a strong sense of necessity.
Rule 6. ‘Must' as a Modal is used to express compulsion or command.
Rule 7. ‘Ought to' as Modal is used to express duty, desirability or moral obligation. Its negative form is ‘Ought not to'. It is not as strong and forceful as ‘must'. It simply indicates that an action is desirable. Must is used to show that speaker orders or expects something to be done.
Rule 8. ‘Ought to have' with a past Participle is used to express a past obligation which was not performed or carried out.
Rule 9. ‘Ought not to have' is used as a Modal to express disapproval of some action carried out in the past.
Examples of rule 1.
1.A teacher must be true to his profession.
2.We must fight for our rights.
3.A judge must be impartial.
4.A good citizen must abide by law.
Examples of Rule 2.
1.We must serve our country.
2.Children must obey their parents.
3.You must respect our teachers.
4.We must be kind to animals.
Examples of Rule 3.
1.We must teach him a lesson.
2.I must have my revenge.
3.We must push back the enemy.
4.We must end corruption.
Examples of Rule 4.
1.He must have passed the examination.
2.She must be married now.
3.The shop must be closed at this time.
4.The sun must have set by now.
Examples of Rule 5.
1.You must call in a doctor at once.
2.He must be ready in an hour.
3.She must weep or she shall die.
4.You must punish the guilty.
Examples of Rule 6.
1.She must reach the class in time.
2.You must give up smoking.
3.They must go home now.
4.You must get out at once.
Examples of Rule 7.
1.You ought to control your diet.
2.We ought to meet more frequently.
3.He ought to look after his aged parents.
4.She ought to serve tea to the guests.
Examples of Rule 8.
1.The driver ought to have applied the brakes.
2.She ought to have kept her promise.
3.We ought to have given them a warning.
4.He ought to have missed a meal.
Examples of Rule 9.
1.You ought not to have kept us in the dark.
2.He ought not to have married an actress.
3.She ought not to have left her home.
4.I ought not to have forgiven him.