must, have to, should
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
Must is used in written rules and instructions
Applications for the job must be received no later than 23 April
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
According to the law, you have to drive with no more than 50 miles an hour
should
You can use should to give advice
You should exercise more if you want to keep fit
You should take vitamins during winter
You can use should when something is not right or what you expect
I wonder where Tom is. He should be here by now
Remember: should is not as strong as must or have to