【一】宾语从句
【1】They knew that he is a child.
变为一般问句：Did they know that he is a child?【正确】
【2】He asked whether you need help.
变为一般问句：Did he ask whether you need help?【正确】
【3】They've known why he hadn't come here.
变为一般问句：Have they known why he hadn't come here?【正确】
备注：宾语为从句的时候，只需要主句一般疑问句，宾语从句表示不变。
【二】表语从句
【1】The fact is that he is a child.
变为一般问句：Is the fact that he is a child?【正确】
【2】The question is whether you saw me.
变为一般问句：Is the question whether you saw me?【正确】
【3】The problem is how we will go there.
变为一般问句：Is the problem how we will go there?【正确】
备注：表语为从句的时候，把主句be动词提前到句首变一般疑问句。
【三】主语从句
【1】That he is careful is a fact.
变为一般问句：Is that he is careful a fact?【错误】
更正：Is it a fact that he is careful?
【2】Whether they will come is a question.
变为一般问句：Is whether they will come a question?【错误】
更正：Is it a question whether they will come a question?
【3】How we go there is a problem.
变为一般问句：Is how we go there a problem?【错误】
更正：Is it a problem how we go there a problem.
备注：that引导陈述分句作主语和wh-疑问词引导疑问分句作句中主语，变一般疑问句的时候需要使用外置主语结构，即使用形式主语it ，把真正主语放在句末。
Warm prompt: The above answer,based upon my personal understanding, is only for reference.
