what he regretted that he had done这是感叹句么?为什么用what

这个改过的句子还是错的.没有动词.另这个句子不是感叹句.是一个由what引导的主语从句
这个句子正确的应该是：what he regretted was that he had done ⋯⋯
what he regret 在句子中做的是主语,在整个句子中充当主语从句.was是这个句子的谓语即系动词.that he had done 是这个句子的表语,在句中充当表语从句.
所以整个句子的结构是：what he regretted（主语）+ was （谓语或系动词）+that he had done （表语）

这个句子应该是；what he regretted was that he had done ⋯⋯
what he regret 做的是主语
这个句子是由what 引导的主语从句