By the time he was twelve, Edison ____ to make a living by himself.
A. would begin B. has begun C. had begun D. was begun(全国高考)
上面的试题要求选 C(had begun)。这是一个考试热点:从句 by the time + 过去时要求主句用过去完成时。但是下面的实例却违背了这个规则:主句用了一般过去时。请解释:
By the time the doctor came, the man was able to sit up, but he still looked pretty sick.
当 by the time 从句用过去时时，主句通常用过去完成时。这时 by the time... 表示“在……之前”“到……的时候”，而主句则表示在此时间之前某个事件业已完成。当从句为一般现在时，而主句为将来完成时，情况亦是如此：
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By the time I got to the station the train had already gone. 我到达火车站时,火车已经开走了。
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By the time he was fourteen years old, he had learned maths all by himself. 到他14岁,他已经自学了数学。
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By the time you come back, I'll have done my homework. 在你回来以前,我的作业会都做
完的。
但是,当 by the time 用过去时时,主句有时也用一般过去时,这时 by the time 常常表示“在……的时候”“到……的时候”。当 by the time 从句用一般将来时,而主句用一般现在时或现在进行时,情况亦是如此:
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By the time we'd walked five miles, he was already exhausted. 当我们走了五英里的时候，他就已经筋疲力尽了。
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By the time her breakfast tray came in Bright was ready for the comfort of food and hot coffee. 等早餐端进来时，布赖特已准备好享受一顿美餐和热咖啡了。
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By the time the alarm went off, I was awake. 当闹钟铃响的时候，我在醒着。
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By the time he was taken to the hospital he was nearly dead. 他被送到医院时已经几乎不行了。
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By the time he was fourteen, he was able to teach himself advanced maths. 到了十四岁，他能自学高等数学了。
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I shall be away by the time (that) the clock strikes twelve. 钟敲 12 点的时候, 我就要走了。
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By the time they caught up with us, they were sopping wet but otherwise none the worse for wear. (Hillary, Living History) 他们赶上我们时，已经全身湿透，不过好在没有受伤。（希拉里，《亲历历史》）