用when时要用时态来表示清楚事件发生的先后顺序

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Tim was cleaning the house when his wife came home. 过去进行时+一般过去时，妻子回家时他正打扫房子。

Tim had cleaned the house when his wife came home. 过去完成时+一般过去时，妻子回家时，他已经打扫好了房子。

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