在下面几种情况下，unless和if not不能随便换用。
（1）当句子表示“B事物如果不停止，A事物将会发生”，我们可以用if not或者unless.
I‘ll come back tomorrow if there's not a plane strike.(OR...unless there's a plane strike.)要不是飞机出故障了，我明天就打算回来。
Let's have dinner out-if you're not too tired.(OR...unless you're too tired.)如果你不是太累的`话，我们出去吃饭吧。
（2）如果表示“A事物发生是因为B事物没有发生”，我们一般只能用if not.
I'll be glad if she doesn't come this eveing.(NOT I'll be glad unless she comes this eveing.)
如果她今天晚上没有来，我会非常高兴。
She'd be pretty if she didn't wear so much make-up.(NOT...unless she wore so much make up.)
如果她没有化如此浓的妆，她会更漂亮。