if和unless的用法区别：unless=if not。

Come tomorrow if I don't phone unless I phone.

如果我不打电话，我明天过来。

I‘ll take the job if the pay's not too low unless the pay's too low.

如果工资不是太低，我将从事这份工作。

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。