1

To talk about rules and strong obligations, we use 'must' and 'have to'. Generally, 'have to' is used for external rules (like laws or company policies), while 'must' is used for personal, internal obligations (like a promise to yourself). For example: 'I have to wear a uniform at work' versus 'I must call my mother tonight.'

2

The negatives are very different! 'Must not' (mustn't) means it is strictly forbidden (e.g., 'You must not smoke here'). 'Don't have to' means it is not necessary, but you can if you want (e.g., 'You don't have to wear a tie').

3

To give advice or a strong recommendation, we use 'should'. It is not a strict rule, but it is a good idea. For example, 'You should see a doctor about that cough.'