Here are the corrected sentences:

If John bought a video recorder, he wouldn’t miss his favourite programmes.
If Peter didn’t eat too many chips, he wouldn’t be overweight.
If she hadn’t gone in the rain last night, she wouldn’t have a cold now.
If he didn’t work twelve hours a day, he wouldn’t be tired.
If Tom were taller, he could reach the shelf.
Unless I were so tired, I would join you there.
Unless I had known the answer, I could have told you.
Were I you, I would study harder.
If the doctors could find out the remedy, they could save a lot of people.
If we used public bus service, we could get rid of pollution in the city.
Were I interested in that subject, I would try to learn more about it.
If I were you, I would sell the car.
If today is Sunday, the pupils can go swimming.
If the boy were not so small, he could go to school.
If this chair were not too heavy, he could move it.
If the church bells didn’t ring, they wouldn’t keep me from sleeping.
If he didn’t smoke, he wouldn’t be so weak.
If I do morning exercise, it makes me feel better.
If we have a fridge, it keeps the food from spoiling.
Why don’t you study hard? => If you studied hard, you would do better.
Why don’t you stop smoking? => If you stopped smoking, you would be healthier.
Without the air, we would die. => If we didn’t have air, we would die.
Without the Sun, man would live in the darkness. => If we didn’t have the Sun, we would live in the darkness.