1. The picnic was cancelled because it rained
==> if  it didn't rain, the picnic wouldn't have been cancelled.
2. Susan felt sick because she ate four cream cakes 
==> if Susan didn't eat four cream cakes, she would've not felt sick.
3. If the weather is fine, we'll go hiking this weekend
==> if we go hiking this weekend, the weather must be fine.
4. If you work hard, you will pass the exam
===> unless you work hard then you'll pass the exam.