This document discusses the use of modal and related verbs to express functions such as necessity, prohibition, permission, advice, ability, and possibility in the present, past, and future tenses. It provides examples of how to use must, have to, need to, mustn't, can't, may not, should, ought to, can, may, could, be able to and will have to/need to in sentences. The document also notes that have to and must are often interchangeable, and that be able to replaces can in non-present tenses to discuss ability.