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In a few years the small work shop had become a large factory which employed seven hundred and twenty –eight people .在此句中为什么要用had become，它在句子中做什么成分，而它又是什么结构呢？If it can be done, it must be done well.中it ,be 可以省略么?为什么？while 和when 就是像引导I was cooking when she was reading the book（我不知道这样作对不对）这一类的句子时，用法和区别都是什么呢？？句子成分请老师详细解答,非常感谢。
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