若非

例句：

1. 1. If it's not raining, we can go for a picnic.
   如果不下雨，我们可以去野餐。
2. 2. If you don't finish your homework by tomorrow, you'll fail the test.
   如果你明天之前不完成作业，你会考试不及格。
3. 3. If he doesn't come back soon, I will call the police.
   如果他不尽快回来，我会报警。