SHOULD: This modal verb is used to give advice.Have a look at the picture.
This man has got a toothache.
What should he do?
Well, he should go to the dentist!
HAVE TO / HAS TO and MUST: These modal verbs are used to express obligation.Have a look at the picture.
These pupils are at school because they have to.
Do you have to go to school? Yes, you do.
Does your teacher have to go to school? Yes, she does. She has to / must go to school because that's her job.
DON'T HAVE TO or DOESN'T HAVE TO: This modal verb expresses lack of obligation.Have a look at the picture.
Do these girls have to wear a uniform at school? Yes, they do. It's obligatory for them.
Do you have to wear a uniform? No, you don't have to wear a uniform because it's not obligatory or necessary.
Your teacher doesn't have to wear a uniform at all!
