Answer: When we want to express permission, prohibition (not allowing something), obligation or no obligation we use modal verbs. Permission – can, may, could. Can is most often used to ask for or give permission but may and could are also possible even though they are not used as often as can
obligation : must, have to, should and ought to. There are two types of modal verbs of obligation ; those that primarily express a firm obligation or necessity – must and have to. those that express a recommendation or moral obligation – should and ought to.
Explanation:
