What _______(happen) here?There is blood on the floor.这里happen为舍要搭配has,直接填happen不行吗?

What _______(happen) here?There is blood on the floor.这里happen为舍要搭配has,直接填happen不行吗?
What _______(happen) here?There is blood on the floor.
这里happen为舍要搭配has,直接填happen不行吗?

What _______(happen) here?There is blood on the floor.这里happen为舍要搭配has,直接填happen不行吗?
HAPPEN有瞬时性
从语法上要不填is happening 要不就是has happened
从句意上 应该是 has happened

因为动作已发生，用一般过去时 happened 或现在完成时 has happened。
http://zhidao.baidu.com/question/328481982.html

因为动作在过去已经发生

what happened曾发生过什么？
what's happened已经发生了什么？
what's happening正在发生什么

事情已经发生在过去了,