The document defines modal verbs as auxiliary verbs that indicate modality or likelihood, ability, permission, or possibility. It provides a list of common modal verbs such as can, could, may, might, will, would, shall, should, ought to, and must. The document discusses the structure of modal verbs, stating that they do not add suffixes for third person singular and do not require auxiliaries to form negatives or questions. Various uses of different modal verbs are defined through examples, such as expressing permission, possibility, decisions, suggestions, requests, and indicating what is required or correct.