1. The school (that / which) he once studied in is very famous.

= The school in which he once studied is very famous.

    此句中，关系代词that/which引导限制性定语从句，指代先行词school，在从句中作介词in的宾语，介词短语(in+that/which)作从句中的地点状语。注意：如果介词放在定语从句末尾，是非正式语体，关系代词用that或者which都可以，也可以省略。如果介词放在定语从句之首，是正式语体形式，关系代词不能用that，只能用which，也不能省略（在定语从句中，关系代词不能放在介词后面）。

    2.The plane may be several hours late, in which case there’s no point in our waiting.

    此句中，which作为关系代词引导非限制性定语从句，代表整个主句内容。which在从句中作定语修饰case，case是介词宾语，介词短语 in which case作条件状语。

    3.Do you still remember the day when (on which) we went to visit the museum together?

    此句中，关系副词when或关系代词which引导限制性定语从句。先行词为the day。 由于定语从句中缺少时间状语，因此要么用关系副词when引导从句，when作从句的时间状语，要么用关系代词which加介词on，由介词短语on which作从句的时间状语。

   【参考】在非正式文体中，表示时间的普通名词后面的when常常由that来替代或省略。例如：I'll never forget the day (that) we met. 我永远不会忘记我们见面的那一天。（摘自《牛津英语用法指南》第三版498.6节）所以，Do you still remember the day when (on which) we went to visit the museum together? 也可以改为：Do you still remember the day that we went to visit the museum together?或者Do you still remember the day we went to visit the museum together?