这个句子是含有定语从句的主从复合句.有以下几种表达形式:
That is the reason( for which) he is leaving so soon .
定语从句中的介词前移,构成“介词+关系代词”.这时要注意:
(1)“介词+关系代词”等于关系副词;
(2)紧跟介词的关系代词不可以省略,不可以被取代;
That is the reason (why) he is leaving so soon .
关系副词在定语从句中作状语,表示原因‘’
That is the reason ( which/that/省略) he is leaving so soon for.
作宾语的关系代词,可以省略.