Решение:
- You must not say anything if you don't want to. (Это правило или запрет, который нужно соблюдать).
- I can hear you quite well. You must not shout. (Это необходимость или запрет, который нужно соблюдать).
- He has left the army and doesn't have to wear a uniform any more. (Это отсутствие необходимости).
- I've bought everything, so you don't need to/don't have to not go shopping. (Это отсутствие необходимости).
- Tim gave me a letter to post. I must not forget to post it. (Это сильная рекомендация или обязанность).
Ответ: 1. must; 2. must; 3. have to; 4. don't need to/don't have to; 5. must.