She didn't tell me the reason,which she said she would keep as a secret forever.为什么用which不用wh

简单地说 在逗号后面的用which 几率比较大

用语法说就是 他是非限制性定语从句

用what就要解释内容了，而后半句she said she would keep as a secret forever并未指reason内容。所以用which

which 引导的定语从句，有逗号所以不能用that引导

定语从句
定语从句（Attributive Clauses）在句中做定语，修饰一个名词或代词，被修饰的名词,词组或代词即先行词。定语从句通常出现在先行词之后，由关系词（关系代词或关系副词）引出。
关系代词有：who, whom, whose, that, which等。
关系副词有：when, where, why等。

which引导的非限制性定语从句