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.

IV. Prohibition

  • MUSTN′T → You mustn′t step on the grass.