Ответ:
1. If I had bought the tickets, I would have gone to the concert.
2. If our relatives had come, we would have spent the weekend together.
3. Parents wouldn't have allowed him to go for a walk with his friends if he hadn't done his home duties.
4. If the rain hadn't stopped, we wouldn't have gone to the picnic.
5. I would have taken you to the airport if I had had a car.
6. You wouldn't have visited the doctor if you hadn't drunk cold milk.
7. She wouldn't have lost her bag if she had been more careful.
8. If the driver hadn't taken the wrong turning, we wouldn't have arrived late.
9. People would have lived in the clean world if they hadn't polluted the environment.
10. If we had got up early, we wouldn't have missed the bus.