This document summarizes the uses of the modal verbs must, have to, and should as they relate to obligation. It explains that must indicates obligation from the speaker's perspective, while have to indicates obligation from another person. Must is more common for specific or personal obligations, while have to is used more for general or external obligations like rules. In spoken English, gotta is sometimes used instead of have to. The document also discusses the differences between mustn't, which indicates prohibition, and don't have to, which means an action is not necessary or required.