1.
a) You needn't walk - why not save your energy? I'll take you in the car.
b) You can't walk - it's freezing cold and you don't have a coat.
2.
a) Beth can bring food to the party, but it's not really necessary.
b) Beth doesn't have to bring food to the party - we've already got plenty.
3.
a) The students have to revise - the exams are in three weeks.
b) The students don't have to revise - the exams finished last week.
4.
a) Katy has to wear formal clothes - the wedding invitation says formal dress.
b) Katy can wear formal clothes, but not everyone is going to be smart.