Here are the sentences using "if" and "unless":
People enjoy their jobs if they find them fulfilling.
- People do not enjoy their jobs unless they find them fulfilling.
It will not rain tomorrow unless the weather forecast changes.
- It will rain tomorrow if the weather forecast indicates so.
He will pass the exam if he studies hard.
- He will not pass the exam unless he studies hard.
You will have an accident unless you drive more carefully.
- You will drive safely if you drive more carefully.
You will get pink unless you mix red and white.
- You will create a different color if you do not mix red and white.
You can buy me some aspirin if you go to the chemist's.
- You cannot buy me some aspirin unless you go to the chemist's.
I will see Jane if you give her your message.
- I won't see Jane unless you give her your message.
You will be late for school unless you hurry.
- You will be on time if you hurry.
Feel free to compare these with your partner!