I. Obligation to do something
- MUST → They must obey the rules.
- HAVE TO → Everybody has to arrive in time.
II. Advice or recommendation
- SHOULD → You should have a rest if you feel tired.// You shouldn′t skip breakfast.
- OUGHT TO → They ought to read more.
III. Lack of obligation
- DON′T HAVE TO → You don′t have to bring your laptop with you.
- MUSTN′T → You mustn′t step on the grass.