The solution for exercise Grammar 2 in chapter 08 - Language focus 1 of Cutting Edge Upper-Intermediate - Student's Book by Sarah Cunningham, Peter Moor and Jonathan Bygrave
Question
Find modals in the website that mean.
Answer
must (have to describes a necessity, whereas must describes a necessity which originates from the speaker)
mustn't
don't / doesn't have to (this describes the lack of a necessity, whereas mustn't describes a negative obligation)