I don't doublt _he is telling the truth.A whether B if C that D which

I don't doublt _he is telling the truth.A whether B if C that D which
I don't doublt _he is telling the truth.
A whether B if C that D which

I don't doublt _he is telling the truth.A whether B if C that D which
因为前面是don't doublt 所以从句用that 引导.
如果前面是肯定或疑问,则用whether

严谨用法呢，A和B都还要加or not。which前面没指代词。只有that引导宾语从句咯。