ObligationHave toMustShould

Strong obligationMust ≈ Only Present Personal / occasional

obligation

Mustn’t ≠Prohibition = can’t / not allowed

Have to (have got to)

All tenses External obligation

(law/rule)

Don’t have to

No obligation (= not necessary)

Weak obligationshould

Modal verb Advice / opinion You should tell her how you feel.

more common in questions, especially in daily conversation

Negative: should not to Past form: should have V-ed(3)

ought to Semi-modal verb Advice / opinion (a little

stronger)You ought to tell her how you feel.

Ought sounds rather formal when used interrogatively

Negative: ought not to Past form: ought to have V-ed(3)

=

SHOULD: GIVING ADVICE MUST/MUSN'T: STRONG OBLIGATION

 1. I don't think you ____ eat so much chocolate. It's bad for your health.2. Children _____ behave well when they're in class.3. If you drive a car, you ____ stop at marked crossings.4. I think waiters _____  try to be pleasant with customers.5. Some workers _____ wear hard hats because their jobs are considered dangerous.6. If you feel sick, you _____ stay at home. It's better for you.7.  Police officers ______ wear a uniform. 

MUST:  STRONG OBLIGATION. IT'S PERSONAL. IT INVOLVES THE SPEAKER'S OPINION.  HAVE TO:  STRONG OBLIGATION. IT'S EXTERNAL: law, rules...8. I _____  do my homework. My teacher always checks if we have done the exercises.9. I _____ get my hair cut. I don't like it so long.10. We ____ go home. My mother has told me to arrive at ten o'clock.11. I ____ study hard. I want to pass the exam.12. In Spain, children ____ go to school until they're 16.13. You _____ phone every day, darling!

MUSTN'T: STRONG PROHIBITION DON'T HAVE TO: IT'S NOT NECESSARY

14. You ____ accept things from strangers!15. When you go into a clothes shop, you _____ have to buy something. You can just look.16. You ____ steal things from shops.17. Children ______ pay bills.18. Children ______ eat junk food every day.19. Some millionaires _____ get up very early.