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)生是因為B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，她會更漂亮。