If you want to focus on must/mustn't as a started, going in with explanations about modal verbs might be overload, and may drag in other verbs and complicate things. Also, using examples that are already part of their knowledge and avoid having to teach lots of vocab may make the must/mustn't message clearer. Also,, think about the questions you will ask, as 'Can you smoke in the cinema?' is more natural than Must you, but then you could answer No, you can't/mustn't.
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