____ he comes, we won't be able to go.
A. Without B. Unless C. Except D. Even(全国高考)
答案为 B。但是我对 unless 的用法和意义不大明了。
unless 是个从属连词,引导条件状语从句。由于 unless 具有否定意义,因此它引导的是个否定的条件(如果……不)。在初级阶段可以把它看作是 if … not 的同义表达手段。例如：
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Unless he comes ... = If he doesn't come ... 如果他不来……
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Unless you work hard, you will fail. = If you don't work hard, you will fail. 如果你不努力工作，你会失败的。
还要注意,unless 从句,如同 if 从句一样,也要用一般现在时代替一般将来时,见上面的试题以及下面的例句:
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“Are you going to try again?” I asked. “Yes, sir. Unless I visit every bookstore in town, I shall not know whether I can get what I want.” “你还要再试一次吗?”我问道。“是的，先生。如果我不到城里的每一家书店去看看，我就不知道是否能买到我想要的书。”
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Unless it rains, I'll pick you up at 6:00. 如果不下雨,我将在6点钟把你接走。
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They can't keep the objects longer than that unless they can prove they're dangerous. (J. K. Rowling, Harry Potter and the Deathly Hallows) 除非能证明那些东西有危险，否则就不能继续扣押。(《哈利·波特与死亡圣徒》)
请做下面的英语试题：
- Most birds find it safe to sleep in the trees, but ____ they have eggs or young chicks, they don't use a nest.
A. why B. how C. unless D. where(全国高考湖南卷,07-33)
- I won't call you, ____ something unexpected happens.
A. unless B. whether C. because D. while(全国高考卷 I,07-26)
- You'll fail in the driving test ____ you have more practice.
A. if B. unless C. when D. after(全国成人高考,92-45)
- You won't pass the exam ____ you work harder.
A. since B. if C. when D. unless(上海会考,92-12)
- We could go to a concert ____ you'd prefer to visit a museum.
A. if B. because C. unless D. since(全国英语专业四级,92-49)
- ____ their policy can be changed the future for that country will be indeed bleak.