I don’t know when it is that he will come back.

特殊疑问句的强调句型 结构式 特殊疑问词+ is/was it that+ 其他部分(陈述语序).
比如 when will you leave?改成When is it that you will leave?
从句的强调句型,只要把上面的is/was it that改成 it is/was that
楼主你说的那些事你自己根据陈述句的强调句型推理的,并不存在!