有一读者问： 有这样一道题：
(1)We'll not arrive there_________the sun rises.
A. before B. until
为什么只能选B?
这里之所以选B是由句子的本义决定的。我们知道，until的意思是“直到……为止”，侧重以后选了until，全句的意思就是“我们到日出时才能到达那里”。until常可与not等否定词连用，再如：
(2)He did not leave his post until he had finished the last operation.
(3)Not until I left college did I realize it.
(4)It was not until I had read over your letter that I understood the true state of affairs.
(5)He paid no attention to my warning until he had an accident.
before的意思是“在……之前的任何时候”，侧重以前。如选before，上述句（1）的意思就变成“在日出以前的任何时候我们都不会到达那里”，这一般不会是句子的本意的。但这并不是说before不可与否定词连用，相反地，它也可与否定词连用，如：
(6)It won't be long before spring comes.
(7)It was not many minutes before he sank into unconsciousness.
注意以上两句的重点皆为before从句，另如：
(8)You can't go before the rain has stopped.
(9)He had not seen me before I saw him.
(10)He never waited to hear my answer before he'd resume his conversation with Bailey.
在上述结构中，有时before亦可代之以until，但二者的意义与侧重点不同。且以上述句（9）为例，其意为：我看见他之前，他没有看见我。如用until代替before，全句则意为：他到我看见他时才看到我。
另外，before和until还有两点不同之处：一是before可移至句首，如上述句（9）即可变为：
(11)Before I saw him he had not seen me.
until如被移至句首，其前则须带not, 如上述句（3）。二是until可用否定结构，如：
(12)I have dirtied my hands at it until they're not fit for clean work.
而before之后则是不可用not等否定词的。
