Teenagers have to get at least nine hours' sleep a night. - to say something is necessary, You don't have to use the alarm on your phone. - to say that something is not necessary, You mustn't worry about your friends. - to say it is very important not to do something, You ought to reduce your screen time at night. - to say that something is a good idea (+should), It's clear you shouldn't use your phone or tablet before you go to sleep. - to say something is a bad idea,

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