the man laughed when he realized what had hanppend

题目

the man laughed when he realized what had hanppend
这句话里有三个四个动词过去式.我明白两个. the man laughed.明白.
when he realized 在WHEN的后面用动词过去式也理解.
what had hanppened.这句过去完成时就有点迷惑了.难道WHAT也可以当连接词用么? 所以连接词后面的短句中的动词也可以过去化.又或者句子本来应该是个 The man laughed when he realized (that) what had hanppend
是不是本来应该有个TAHT在WHAT前面的.而书中省略掉了.
分都问问题问完了.劳烦高手不吝赐教

答案

when he realized what had hanppend是个状语从句,what had hanppend是宾语从句,作为动词realized的宾语,因为realized用了过去式,所以其宾语要用过去完成时.也就是一个句子由含宾语从句的复合句构成.在宾语从句中,that可以省略.