在下面幾種情況下，unless和if not不能隨便換用。

　�。�1）當(dāng)句子表示“B事物如果不停止，A事物將會(huì)發(fā)生”，我們可以用if not或者unless.

　　I‘ll come back tomorrow if there's not a plane strike.(OR...unless there's a plane strike.)要不是飛機(jī)出故障了，我明天就打算回來。

　　Let's have dinner out-if you're not too tired.(OR...unless you're too tired.)如果你不是太累的話，我們出去吃飯吧。

　　（2）如果表示“A事物發(fā)生是因?yàn)锽事物沒有發(fā)生”，我們一般只能用if not.

　　I'll be glad if she doesn't come this eveing.(NOT I'll be glad unless she comes this eveing.)

　　如果她今天晚上沒有來，我會(huì)非常高興。

　　She'd be pretty if she didn't wear so much make-up.(NOT...unless she wore so much make up.)

　　如果她沒有化如此濃的妝，她會(huì)更漂亮。