Modals are special verbs that behave irregularly in English and provide additional information about the function of the main verb that follows. They never change form and are always followed by an infinitive without "to." Modals are used to indicate modality and allow speakers to express certainty, possibility, willingness, obligation, necessity, and ability. Common modal verbs include will, shall, would, should, ought to, must, may, might, can, could, have to, and don't/doesn't have to.