modal verb
1
BAD : People are not as careful as they must be and drop their litter in the streets.
GOOD : People are not as careful as they should be and drop their litter in the streets.
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Use must (or have to ) when, for example, there is a law or rule and you are not free to choose or decide for yourself: 'Candidates must answer all the questions in Part A and two questions in Part B.'
Use should (or ought to ) when, for example, someone advises you to do something but you are free to choose or decide for yourself: 'At the end of the examination, you should check your answers.'
2
BAD : The pupils mustn't go to the meeting if they don't want to.
GOOD : The pupils needn't go to the meeting it they don't want to.
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Use must not/never when you mean that it is essential that someone does not do something: 'The door to the X-ray room must never be opened when the red light is on.'
When you mean that it is not necessary for someone to do something, use needn't or don't need/have to : 'You needn't pay now. You can wait until the furniture has been delivered.'
3
BAD : You must be pleased to hear that I've already got the tickets.
GOOD : You'll be pleased to hear that I've already got the tickets.
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When you inform someone of something, use will be + pleased/interested + to hear/know/learn (NOT must ): 'You will be pleased to know that your old friend Peter has been promoted to Associate Professor.'
4
BAD : If you can't find her, she must hide somewhere.
GOOD : If you can't find her, she must be hiding somewhere.
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When you do not actually know where someone is or what they are doing, but certain facts allow you to guess, use must be or must be doing : 'If she isn't in her office, she must be in the canteen.' 'If she's in the canteen, she must be having her lunch.'