692　unless和if…not

① unless是从属连词，为书面语，用于引导条件状语从句，意为“除非，如果……不”。unless本身具有否定意义，因此所引导的从句谓语动词用肯定式，相当于if…not，常可换用。unless从句常位于句尾，表示强调时位于句首。

Don't come unless I call you. 我不打电话你就不要来。

=Don't come if I don't call you.

Unless you work hard，you will fail. 除非你努力工作，不然你会失败的。

= If you don't work hard，you will fail.

② unless只能引导真实条件句，用直陈语气，不可用虚拟语气；而if…not既可以引导真实条件句，也可以引导非真实条件句，用虚拟语气。因此，当if…not引导非真实条件句，谓语动词用虚拟语气时，就不可换用unless从句。

She will come unless it rains. 如果不下雨，她就会来。（真实条件句）

=She will come if it doesn't rain.

如果你早来十分钟，你就会赶上火车了。

If you had come ten minutes earlier，you would have caught the train.

（非真实条件句，虚拟语气）

Unless you had come ten minutes earlier，you would have caught the train.

③ unless如果所引导从句的谓语动词是否定式，就不可转换为if…not结构。

这本词典是值得买的，除非你没有足够的钱。

This dictionary is worth buying unless you don't have enough money.

This dictionary is worth buying if you don't have enough money.

④ 当从句的动作或情况不发生，主句的动作或情况才可发生时，要用if…not，不可用unless。

We'll be surprised if he doesn't do it that way. 要是他不那样做，我们倒会感到奇怪了。

⑤ unless和if…not引导的从句，要用一般现在时代替一般将来时。

除非明天下雨，不然我们将去度假。

We shall go on a holiday unless it rains tomorrow.

We shall go on a holiday if it doesn't rain tomorrow.

⑥ unless如果所引导从句的主语和主句的主语相同，谓语中又有be动词时，从句的主语和be动词可省去。

She won't come unless invited. 如果不邀请，她不会来。（省去she is）