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。。。