Must is used to express something that is necessary or obligatory according to rules or instructions. Have to expresses something that is necessary or obligatory. Should is used to give advice or suggest something that is recommended or expected but not compulsory.

1. must, have to, should

2. must You can use must to give your opinion or say that something is necessary I haven’t spoken to my mother for ages. I must phone her

3. Must is used in written rules and instructions Applications for the job must be received no later than 23 April

4. have to I have to do something – It is necessary to do it. I am obliged to do it You have to be over 21 to drink alcohol in the USA

5. According to the law, you have to drive with no more than 50 miles an hour

6. should You can use should to give advice You should exercise more if you want to keep fit

7. You should take vitamins during winter

8. You can use should when something is not right or what you expect I wonder where Tom is. He should be here by now

9. Remember: should is not as strong as must or have to