The document discusses the uses of the modal verb "must". It can express personal obligation from the speaker's perspective but is not a real obligation. "Must" can also refer to actual obligations from rules or laws. It is generally used for present or future tense, while "have to" refers to past tense obligations. "Must not" and "mustn't" express prohibition - something that is not allowed or permitted, whether as an opinion or actual rule. "Must" is followed by a main verb in the base form without "to".