The document discusses the meanings and uses of the words "must" and "mustn't" in English. "Must" expresses obligation or necessity from an external source. Examples are given of sentences using "must" to express things that are required or obligatory. "Mustn't" expresses prohibition. More example sentences are provided using "mustn't" to indicate things that are forbidden or not allowed. Students are given exercises to practice forming sentences using "must" and "mustn't" correctly.