【问】 EED对介词by的释义是：If something happens by a particular time，it happens at or before that time. 对介词until的解释是：If something happens until a particular time，ithappens before that time... If something does not happen until a particular time，it doesnot happen before that time and only happens after it. 由此可见，by 1997和until 1997都
 可理解为1997年前。这是不是意味着它们在语义和用法上没有什么区别呢?
 【答】 虽然介词by和介词until都表示句子所描述的情景发生在by短语和until短语所指的时间之前，但是它们所指的具体时间和用法却是不相同的。by twelve o'clock是指9点、10点、11点或12点，而until twelve o'clock则是指12点。在用法上，until用在肯定句中时，要求谓语动词必须是持续动词(be，live，like，love，wait)，而by却不是这样。因此，下面使用until的句子都是错误的(应将until改为by)
 ×You'll have to leave until Monday midday at the latest.
 ×I will have finished this book until tomorrow.
 ×Until the middle 1960s，the national networks were broadcasting most of their programsin color.
 从表意上看，until表示主句的情景在时间上持续到某个时刻为止。因此，在肯定句中until(till)只能与持续动词连用。而by则表示句子的动作发生在某个时刻或某个时刻之前。因此，它只与瞬时动词(momentary verbs)连用。
 请对比：
 (a) The troops remained there until midnight.
 部队在那里一直停留到午夜。
 (b)×The troops remained there by midnight.
 (a) My girlfriend worked there till Christmas.(CGEL)
 我的女朋友一直工作到圣诞节。
 (b)×My girlfriend worked there by Christmas.
 (a) She arrived by Christmas. (CGEL)
 她圣诞节前到达。
 (b)×She worked by Christmas.
 类例：
 1. I will have retired by the year 2020.
 到2020年我将退休。
 2. I have to keep writing until the end of next year.
 我得一直写到明年年底。
 3. My book will be finished by the end of next year.
 我的书明年年底前写完。