we were curious to find out what had happed.这里的what是是用法,为什么这样用

we were curious to find out what had happed.这里的what是是用法,为什么这样用

这是代词what的一种特殊用法.即这里的what=the things that,what在宾语从句中做主语.
we were curious to find out what had happed=we were curious to find out the things that had happed.

what happens 主语+ 谓语 ：发生了什么事， 事情作主语，

上面句子是过去完成时：what had happened

该句是宾语从句句式。即what had happed 作find out的宾语。what 又做从句中的主语。