This is the negative form of "must," but unlike other verbs, the negative does not mean "not necessary." It means "forbidden."
| Function | Modal Verb(s) | Strength | Example | |----------|--------------|----------|---------| | | must, have to | High | You must stop at a red light. | | Internal obligation | must (self-imposed) | High | I must call my mom today. | | External obligation | have to (rules / others) | High | She has to wear a uniform at work. | | No obligation | don’t/doesn’t have to | Neutral | You don’t have to pay for water here. | | Prohibition | must not / mustn’t | Absolute | You must not smoke in the hospital. | | Permission (give) | can, may | Medium/Formal | You can leave early today. | | Permission (ask) | can, could, may | Informal → Formal | Could I open the window? | | Lack of permission | can’t, may not | Medium/Formal | Students may not use phones. | Modal Verbs Of Obligation Prohibition And Permission Pdf
For further study, comprehensive guides are available from resources like the British Council Test-English based on these modal verb rules? Modals: permission and obligation - British Council This is the negative form of "must," but