6. If we turn off the tap when brushing the teeth, we'll save a lot of water.
7. If people travel more by public transport, the harmful gases in big cities'll be reduced.
8. If we start at 6.00, we'll arrive there before soon.
9. If the boy eats so many green grapes, he'll be ill.
10. If we use reusable bags for shopping, we'll reduce waste.
XXIII
1. Linh's parents are proud of him because he always gets good marks.
2. We're very interested in playing soccer when living in the countryside.
3. I haven't talked to my uncle since he bought a new house in the city centre.
4. The Browns bought a lot of food because they're going to have a party.
5. It's only a small car so there isn't enough room for all of us.
XXIV 
1. The girls are playing tennis at the moment.
2. John always goes to school by car.
3. What do you do in the afternoon?
4. They are chatting in the class now.
5. My sister doesn't buy a new car.
6. What do you think about the role of robots in the past?
7. Home robots can do the housework.
8. Nurse robots can help sick people.
9. Worker robots can build big buildings.
10. Teacher robots can teach in classes.