关于in which.语法高手请进.这句话： He had at last been allowed to send a fax in which he informed the edior that he had been arrested.请问这句话里面 定于从句引导词 which前面为什么要加 in 呢?请问这是什么语法现象? 什么时候 引导词前面要加介词呢? 请详细说明并举例子? 谢谢~! 可酌情加分哦~!一定要解释详细些.

in which 属于定语从句中关系词的一类，介词+关系词。其中的介词与句中其他成分有一定的逻辑关系，所以不能省略。
看怎么变化的：
He lives in a city.The city is far away.
→The city he lives in is far away.
→The city in which he lives is far away.
同样的,a fax in which he informed the edior that he had been arrested
还原之后就是 he informed the edior that he had been arrested in the fax。
因为是在传真中，所以in不能省，这样就比较好理解了。

这个句子是定语从句。
首先明白定语从句的概念，定语从句有可能去掉（介词+）先行词之后是一个完整的句子，但是意义不完整，与主句扯不上任何关系了。
he informed the edior that he had been arrested。是一个的句子，但是却与fax没有了任何关系。
若要与之发生关系，则
he informed the edior that he had been arrested in a fax（在一封传真中）.a fax是先行词，用which代替同时in也要提前（如果没有in的话这个句子是不是不完整了啊he informed the edior that he had been arrested a fax，这就是一个错句）。这里的in不就是在....里面吗。
例句：
This is the river in which he can swim.

翻译：最终他被允许可以发一份传真来告诉编辑他已经被捕了.用IN主要是要看从句中的意思,WHICH这里指代的是FAX里讲的内容,如果把WHICH 去掉,我们可以看从句如下：in the fax he informed the editor that he had been...

in which 就是where，意思就是说在这封邮件里他说他被逮捕了。很简单啊。
for example I had a dinner at KFC in which (where) i spent 20 yuan.