A modal auxiliary verb, often simply called a modal verb or even just a modal, is used to change the meaning of other verbs (commonly known as main verbs) by expressing modality—that is, asserting (or denying) possibility, likelihood, ability, permission, obligation, or future intention.
Modal verbs are defined by their inability to conjugate for tense and the third person singular (i.e., they do not take an “-s” at the end when he, she, or it is the subject), and they cannot form infinitives, past participles, or present participles. All modal auxiliary verbs are followed by a main verb in its base form (the infinitive without to); they can never be followed by other modal verbs, lone auxiliary verbs, or nouns.