24. If his cousin doesn't drive too fast, he won't get an accident.
→ Unless his cousin drives too fast, he won't get an accident.

25. Unless the house is small, they will rent it.
→ If the house isn't small, they won't rent it.

Ex2:

1. Hurry up or you will be late.
   → If you don't hurry up, you will be late.

2. Be careful or the boy will fall down.
   → If you're not careful, the boy will fall down.

3. Stop talking or we won't understand the lesson.
   → If you don't stop talking, we won't understand the lesson.

4. Water this tree or it will die.
   → If you don't water this tree, it will die.

5. Don't drive too fast or we will get an accident.
   → If you drive too fast, we will get an accident.

6. Don't be late or she will be punished.
   → If you're late, she will be punished.