Sometimes the easiest thing to do with a class is just announce that you're going to spend some time on a particular grammar point. Other times, though, it's better to let the point come up in what is hopefully a realistic context. Of course, coming up with that realistic context can be hard for the teacher. One context I've come up with for looking at 'have to' and 'can't' is the procedure used to get drivers' licenses and rules of the road. Everybody loves complaining about burocracies, so there is also the chance to open up a wider discussion and compare the regulations of different countries.
Simply make a list of the various rules offered by students, using either "You have to..." or "You can't...". Students then can manipulate the forms by asking questions etc., and elaborate for homework by researching other, possibly silly, rules.
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