A modal verb is a type of verb that is used to indicate modality -- that is: likelihood, ability, permission and obligation. Examples include the English verbs can / could, may / might, must, will / would and shall / should. In English and other Germanic languages, modal verbs are often distinguished as a class based on certain grammatical properties.
what is the function of modal auxiliary verbs in english
to indicate modality -- that is: likelihood, ability, permission and obligation