The reason why he didn't come was that he was ill.
The reason why ... is that...是个固定句型,表示,...的原因是...,那么是否意味着 is 前面的句子是主语从句,而that引导的句子是表语从句呢?可否这么理解。
看成分划分:
The reason (主语)why he didn't come (定语从句)was (系动词)that he was ill.(表语从句)
主语部分含有一个定语从句,而不是主语从句。注意概念不同。
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