Hello,

I'm trying to remove a coupling nut from the toilet supply line. Every time I try to unscrew it, the fill valve turns with it. I tried holding onto the fill valve at the same time but it wasn't sturdy enough. Then I tried using a plier to hold onto the fill valve but was afraid to break the fill valve. I've also tried spraying WD-40. Any suggestion would be greatly appreciated. Thank you!

You have to be so careful doing what you are doing.
the only reason I can think of that the nut does not turn is that it is white metal (real crap)
take a hacksaw blade and try to cut up and down on the nut. cut right thru to the threads on the fill valve.
then with a small screwdriver gently ( I do mean gently) tap into the cut you have made to split the nut.
WARNING.. excess heat, excess force etc. will crack the toilet tank.
plan on replacing the fill valve, they are cheap..

They are cheap, so why not just cut the end of the fill valve off just above the nut?

that would work but then there may be trouble getting the fastening nut off which would become a second problem.
of course it might spin right off also ?