Definitions for "Modal verb"
Keywords:  verb, auxiliary, aren, dare, ought
Modal verbs are auxiliary verbs that aren’t used to make questions and negatives, but instead are used to express ideas such as obligation (You must go home now), ability (I can swim), and permission (You may go to the toilet).
Verbs which express the mood of another verb: will/would; shall/should; may/might; can/could; must, ought, need, dare, used to.
see verb.