11. If Peter hadn't been lazy, he wouldn't have failed the exam.
12. If she had known that they were out of work, she would have helped them.
13. If he had seen her, he would have said "hello" to her this morning.
14. If the weather were bad, we could go for a walk.
15. If this exercise weren't difficult, she could do it.
16. If I had enough money, I could buy this new car.
16. If he doesn't get up early, he will miss the train.
18. If you don't hurry up, you'll be late for school.
19. If it hadn't rained heavily yesterday afternoon, the football match wouldn't have been cancelled.
20. If the river hadn't been careless, the accident wouldn't have happened.
GOOD LUCK!