Using subordination:He didn't know what had happened. I didn't know what had happened.
【正确答案】正确答案：Both he and I didn’t know what had happened．
【答案解析】解析：译文：他不知道发生了什么。我不知道发生了什么。合并在一起意思为：他和我都不知道发生了什么。两句为并列关系，可以用both．．．and．．．。