This document discusses the proper uses of various modal verbs in English including must, have to, mustn't, need, should, may, might. It explains that must is used for obligations from the speaker's authority, while have to is used when the obligation comes from an outside source. Mustn't is used to express a prohibition, while don't have to and don't need to express a lack of obligation. May, might and could are used to discuss present or future possibilities. Finally, should and ought to are used to give advice or discuss obligations and what is proper or good.