2015年GCT英语精彩语法复习:时态7
过去进行时过去进行时是由助动词be的过去式加上现在分词构成的，主要表示过去某个时刻正在进行的动作。
At that time she was working as an assistant in a physics labatory.那时她在一家物理实验室做助手。
They took us to see the chemical fibre mill that they were building. 他们带我们去参观了他们正在修建的化纤厂。
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