3 when、where引导的定语从句前有介词的情况
网上的说法：
在定语从句中when、where能代替介词+先行词构成的介词短语在定语从句中作状语。如果用介词+which替换when、where，which代替先行词与介词构成介词短语在定语从句中作状语。
But it was a time when there were many great philosophers.
1. 问题：上面这句话中when可以被at which替换么？把a time还原到从句中是at one time？
2. 看语法书中说when和where前也可以有介词，when前可以与from、since、after等引导定语从句，where前可以与from引导定从：
A. She went to America two years ago,since when I haven't heard from her any more.
B. She climbed up to the top of the hill,from where she could have a good view of the whole town.
问题：比如AB两个句子。when和where在定从中直接就可以作状语呀（代替介词+现行词在定从中作状语），为什么前面还要用介词呢，单独就用when或where为什么不行呢？如果前面有介词应该是since which、from which呀？
最佳答案 2017-11-22 00:16
But it was a time when there were many great philosophers.
1.when=during which
2. 看语法书中说when和where前也可以有介词，when前可以与from、since、after等引导定语从句，where前可以与from引导定从：
A. She went to America two years ago,since when I haven't heard from her any more.
首先把这个句子还原成并列句：
She went to America two years ago, and since then I haven't heard from her any more.
时间副词指代two years ago. 这是二个并列分句表达的信息同等重要。但说话人为了突出第一分句的信息，第二分句就用关系副词when取代时间副词then, 同时去掉并列连词and, 第二分句就成了一个非限制性定语从句，主句表到重要信息，从句表达次要信息，起补充说明的作用。when固然可以作状语，但其意义指代的是两年前那个时间，而定语从句的谓语表示的是自从两年前到现在这段时间，因此必须用介词since。when也可以改为which time. since which time。。。
B. She climbed up to the top of the hill,from where she could have a good view of the whole town.
和上一个句子类似，如果是并列句的话，则为：
She climbed up to the top of the hill,and from there she could have a good view of the whole town.
为了突出第一分句，不想让第二分句喧宾夺主，将地点副词改为关系副词，去掉并列连词and，就得到一个非限制性定语从句，表示次要信息。where指代the top of the hill. 这个地点是向别处看的位置。而不是看山顶，即从山顶往下看，可以清楚的看到山下的城镇。因此必须用form。 where可以改为which place，from which place。。。
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