Prohibition: Mustn't, Can't

We use 'must not' (mustn't) and 'cannot' (can't) to talk about something you aren't allowed to do or you're strongly advised not to do.

For examples,

• You mustn't lean out of the window.
• You mustn't smoke in the bus.
• You can't cross when the light is red.

For strong prohibition we use 'must never'; for examples,

• You must never walk on the railway line.

'Must not' is used to talk about prohibition; however, 'don’t have to' is used to show lack of necessity.

For examples,

• We don't have to work on Sundays.
• I can hear you. You don’t have to shout.