Modals are special verbs that express ability, permission, possibility, necessity, or obligation. They help show the speaker's attitude or opinion about an action. Common modals include can, could, may, might, will, would, shall, should, must, and ought to.
Unlike regular verbs, modals never change their form and are always followed by the base form of the main verb.
Key Rules: First, modals never take 's' in third person singular. You say 'He can swim' not 'He cans swim'. Second, questions are formed by moving the modal before the subject: 'Can you help?' Third, negatives add 'not' after the modal: 'cannot', 'should not'.
Fourth, modals are followed by infinitive without 'to' (except ought to).