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in which引导定语从句，在从句中不作地点状语，而是作方式状语时，用in which；引导定语从句，介词in与从句中某些词语构成固定搭配，用in which。
既能用where又能用in which的情况
引导定语从句，在定语从句中作地点状语，从句与先行词的搭配缺少介词in时，既能用where又能用in which。
（1）This is the house where/in which he used to live.
这是他过去住的房子。
（2）The old school has now turned into a large modern factory where/in which a lot of coulour TV sets are produced every year.
这所旧学校现在已变成一个大型的现代化工厂，每年生产许多彩色电视机。
只用in which不用where的情况
1.引导定语从句，但在定语从句中不作地点状语，而是作方式状语时，只用in which，不用where。
（1）This is the best way in which we can do the work well.
这是我们把工作做好的最好办法。
2.引导定语从句，介词in与定语从句中某些词语构成固定搭配时，只用in which不用where。
（1）He left his work in which he had been very successful.
他离开了他非常成功的工作。