__he couldn't understand was__,as a teacher of chinese,he was asked to teach history.
为什么答案是what;that而不是what;why 如能赐教 感激不尽
为什么不可以是"what he couldn't understand was why,as a teacher of chinese,he was asked to teach history.
表语从句也可以用why引导呀 why在从句中做状语 无可非议?