Can, could, might, may, must, shall, should, ought to, will, would and needn’t are modal verbs. Modal verbs have more than one meaning in English. They can be used to possibility / probability, to express meanings such as obligation, ability, etc. and to making suggestions, etc.
Modal verbs have no –s in the singular and are followed by the infinitive without do e.g.
Could you repeat that?
You shouldn’t drive so fast.
Have to/have got to, need to, be able to are main verbs that can also be used to express obligations/ necessary/necessity And ability.
Past modal are formed by using the modals followed by have (not) and the past participle. e.g.
She must have forgotten.
He shouldn’t have said anything.
Could you have left it in the office?
7.2 Ability
Can and be able to are used to express ability; be able to can be used in various forms (e.g the past, future, infinitive, ing forms)