1. He can't go out bacause he has to study for his exam.
=> If he didn't have to study for his exam,he could go out.
2. She is lazy so she can't pass the exam.
=> If she weren't lazy, she could pass the exam.
3. He will pay me tonight; I will have enough money to buy a car.
=> If he doesn't pay me tonight, I won't have enough money to buy a car.