Modal verbs are verbs that act like auxiliary verbs. For example: can, could, be able to, may, might, shall, should, must, have to, will, would, etc.
Example:
I will go.
-Will I go?
She should leave now.
-Should she leave now?
The modal verbs act like an auxiliary, and they invert with the subject to make a question, to make a negative sentence and to make tag question.