This document discusses modal verbs in English. It defines modal verbs as auxiliaries that add meaning to sentences and accompany the main verb. Some common modal verbs are can, could, may, might, will, would, shall, should, ought to, must, cannot, must not. The document provides examples of how different modal verbs are used to express ideas like ability, permission, possibility, advice, necessity, obligation, prohibition, and certainty. It also discusses the use of modal verbs with perfect infinitives to talk about past situations.