Modals of obligation and prohibition
There are many rules in our lives, at work, at school, at home, in the public places, on the roads, etc., which have been established to ensure order and safety. We use modal verbs to express obligations and prohibitions, or lack of obligation.
Modal verbs have the following grammatical structure: Subject + Modal Verb + Basic Verb
Example:”I must go to bed early tonight”.
And this is how we express prohibition and lack of obligation:
Be careful: there is a big difference in the meaning between ”mustn’t” and ”don’t have to”