Modals and prohibition
From the diagram, we can see that to talk about prohibition there are five options, depending on the situation. ‘Mustn’t’ is the strongest structure, while ‘shouldn’t’ is the weakest. ‘Can’t’ and ‘may not’ relate to medium levels of prohibition. Look at the diagram and then look at the examples below for some more information.
Passengers can’t enter the cockpit during flights.
You may not put your bag under the seat in a beside and emergency exit.
You shouldn’t drink your own alcohol on board.
You mustn’t smoke on board.
We can’t have a day off next Monday.
They may not land after 2am, the airport will be closed.
He shouldn’t perform the landing for the next flight, it’s not his turn.
They mustn’t switch seats after the planes takes off.
He told us we couldn’t switch our flight.