That is the reason that they are all against the plan.

问题描述:

That is the reason that they are all against the plan.
reason后的that为什么不能用why代替?(原句肯定对)

不可以
that引导reason的同位语从句
原句中“他们都反对这项计划”就是这个原因
翻译为“原因就是他们都反对这项计划.”
why引导的是定语从句
翻译为“那就是他们都反对这项计划的原因.”