It took me a long time to understand where he came from.
这句话中where he came from为什么不是主语补语从句,而是宾语从句?
这句话中a long time是宾语,me是宾补吗?
It是形式主语, took是谓语动词, me 和 a long time都是谓语动词的宾语, to understand where he came from是动词不定式短语作真正主语。to understand 是及物动词,所以 where he came from是宾语从句。
宾语和宾语补足语通常存在逻辑上的主谓关系,a long time和me是这种关系吗?
这句话中it是形式主语,真正的句子结构是To understand where he came from took me a long time.首先这个句子,took是谓语,me是宾语,a long time 是宾补。To understand where he came from这一段是动词不定式做主语,where he came from相当于understand的宾语,是宾语从句。