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.
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.'
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.
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.'
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.
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.'
BAD : If you can't find her, she must hide somewhere.
GOOD : If you can't find her, she must be hiding somewhere.
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.'