A modal verb is a special type of verb.  Modal verbs are used to show the level of possibility, indicate ability, show obligation or give permission.Modal verbs behave differently to ‘ordinary’ verbs.

The most common modal verbs are: could, may, can, must, might.

      

https://agendaweb.org/verbs/modals-exercises.html

https://www.english-grammar.at/online_exercises/modal-verbs/m009.htm