Here are the rewritten sentences using the correct mixed conditional form:

1. She didn’t prepare well for the exam, so she didn’t pass.
→ If she had prepared well for the exam, she would have passed.

2. If you don’t study hard, you will fail the exam.
→ Unless you study hard, you will fail the exam.

3. I don’t have enough money, so I can’t buy a new car.
→ If I had enough money, I could buy a new car.

4. He didn’t practice enough, so he isn’t ready for the performance.
→ If he had practiced enough, he would be ready for the performance.