#15
1. mustn't
2. mustn't
3. must
4. can
5. have to
6. can

#16
1. Can I leave the lesson early?
2. You should carry some insect repellent when you trek through the jungle.
3. Pete can skate very well.
4. You don't have to dust the furniture every day.
5. We must make our beds every morning.
6. You mustn't park your car in a bus lane.
7. You shouldn't spend so much time playing video games.

#17
1. George can't dance very well.
2. Can I sit here?
3. You have to stop at a red traffic light.
4. We mustn't keep pets in the block of flats where I live.
5. You should always use a zebra crossing when you want to cross the street.
6. You can take your bike on the London underground.

#18
1. C
2. A
3. A
4. C
5. C
6. B
7. A
8. C