此where非彼where,二者不可混淆。She didn't know where to go. 中,where是疑问副词,where to go是名词性疑问分句的一种简略,句子等于She didn't know where she should/could go. where引导的是疑问句。而 The farmer used wood to build a house in which to store grains.中,in which to store grains是定语从句的一种简略形式,其中的which是关系代词,不是疑问词!句子等于,... to build a house in which he would store grains. 介词+which/whom+to do不定式是一种正式用法,只能是介词+which或whom+不定式,不可以用关系副词,介词必须位于关系代词之前。这种结构只能作定语,只见于书面语,从不会用于口语。所以,你必须区分wh-词是疑问词还是关系词,不能稀里糊涂地觉得差不多。
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