Sure! Here’s a rewritten version of those sentences:

1. You will get bad marks if you don’t study harder.
- **Unless** you study harder, you’ll get bad marks.

2. He can’t help you move by car unless this man is a driver.
- **If** this man is a driver, he can help you move by car.

3. He can’t improve his writing skill unless he practices writing every day.
- **If** he practices writing every day, he can improve his writing skill.

4. You’ll have to pay a fine unless you return this book to the library today.
- **If** you return this book to the library today, you won’t have to pay a fine.

5. You might fall ill unless you go out more often.
- **If** you go out more often, you might not fall ill.

6. Your file will be removed unless you fill in enough information.
- **If** you fill in enough information, your file will not be removed.

7. She will fail this contest unless she tries her best.
- **If** she tries her best, she won’t fail this contest.

8. He wouldn’t be absent from the workshop unless he didn’t have a stomach.
- **If** my brother had a stomach, he wouldn’t be absent from the workshop.

9. I’ll have to report it to the police unless you return the money to the missing person.
- **If** you return the money to the missing person, I won’t have to report it to the police.

10. You cannot gain useful knowledge unless you read books regularly.
- **If** you read books regularly, you can gain useful knowledge.