Do you know what was happened to him.哪里错了?要怎么改?

Do you know what was happened to him.哪里错了?要怎么改?
Do you know what was happened to him.
哪里错了?要怎么改?

Do you know what was happened to him.哪里错了?要怎么改?
Do you know what happened to him.
happen是不及物动词,不能用被动的,所以把was去掉就可以了

后面不能用被动语态
Do you know what happened to him？

was去掉