Modal verbs are used to express permission, ability, obligation, advice, and criticism. They have only one form except for "have to" which changes form based on person and tense. Modals are never followed by the full infinitive. Common modals include can, could, may, be able to, should, ought to, must, and have to. Each modal verb has specific meanings and uses when expressing ability, permission, advice, obligation, and criticism in both the present and past tense.