This document discusses the structure and uses of modal verbs in English. It outlines the rules for their structure, including that they do not take suffixes, are followed by the bare infinitive, and come before the subject in questions. It then discusses their common uses to express obligation, necessity, absence of necessity, prohibition, and advice. The main uses covered are must and have to for obligation, should/ought to for weak obligation, don't have to for absence of necessity, mustn't for prohibition, and should for advice.