Here are the sentences rewritten:

5. David has many accidents due to his carelessness.
=> David wouldn’t have so many accidents unless he is careless.

7. It’s unfortunate that Mary is not here.
=> I wish Mary were here.

8. Sandra might arrive a bit early; she can assist with preparations.
=> If Sandra arrives early, she can help you get things ready.

9. There are no rivers in my village.
=> I wish there were a river in my village.

10. She is not quite tall.
=> She wishes she were taller.

11. I wish I had more money to buy the car because it’s a pity that I don’t have enough.

13. Lam enjoys living quite far from the school.
=> I wish Lam didn’t live so far from the school.

14. I have few friends.
=> I wish I had more friends.

15. The bus is always so crowded; I don’t take the bus to school every morning.
=> If the bus weren’t so crowded, I would ride it to school every morning.

17. We can’t finish the game because it’s raining.
=> If it weren’t raining, we could finish the game.

25. I regret not being able to go with you.
=> I wish I could go with you.

26. I can’t play the piano very well; it’s a pity.
=> I wish I played the piano better.

30. I don’t know how to access the Internet.
=> I wish I knew how to get access to the Internet.

32. If you work hard, you will succeed in passing the exam.
=> Work hard and you’ll pass the exam.

33. You would pass the exam if you studied harder.
=> Unless you study harder, you will fail the exam.

35. Tung would not have an accident now if he drove carefully.
=> Unless Tung drives carefully, he will have an accident.

36. Lan will miss the meeting if she doesn’t start working now.
=> Unless Lan starts working now, she won’t be able to attend the meeting.