I have been trying to figure this out by comparing Hebrew binyanim to Arabic and Akkadian conjuations. But I cannot figure it out.
Is Nif'al a natural part of the G-stem (like pa'al) or has it "wandered" there over the millenia when the original passive and reflexive G-stem conjugations died out? Where did it come from?
Nif'al just doesn't seem very equivalent to Hitpa'el even though as a reflexive of pa'al it should be.