Modal verbs are auxiliary verbs that express ideas like
They are always followed by the simple form of a verb (infinitive).
Examples of modal verbs are: should have / might have / must have / could have
Ability
Lets start with modals of ability. We use can, be able to and could to show that someone has (or doesn’t have) an ability to do something.
Example:
Alan can swim well.
Paul could speak Chinese when he was a child.