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。