I am trying to replace the shut-off valve to the toilet but the old nut onthe inlet pipe will not come off--and of course it does not match threads with the new valve--So I am stuck--and no water to the house!
what inlet nut?? On the shutoff exit? or the tank filler vavle under the tank?? OR do you have compression BEFORE the shutoff? ANY nut that doesnt come off here,,,gets "2" hammers tapping at same time from BOTH sides at once and tapped around as far as you can,,,that USUALLY knocks the crap out of them and loosenes the thread,,,same thing for pipe fittings and threaded pipe etc.!!! hasnt failed to work yet,if not successful, try try again!! Care must be used to govern tapping force to type and size of pipe etc used on,,,dont egg fittings your planning on using again!!(or unscrewing for that matter)
AND what type of pipe are you using(working with)??Are you SURE its threaded?? When all else fails,,, a hex sided line wrench works wonders over a 2 surface open end wrench
Sounds like you may have the classic old brass ferrule ring compressed into the pipe which will not allow the old shut-off valve nut to come off either.
Most all compression shut-off valve fittings should be compatible if it is for 1/2" copper pipe. I usually take the easy road in this situation and just use the old ring and nut on the supply pipe end of a new valve. If for some reason your new valve isn't compatible with the old nut, get one that is. The ferrule rings seal compression fittings, not the threads, so you might want to smear a little dab of clear silicone caulk around that old ring before making it up to the new valve.
Use a backup wrench on the new valve to tighten the old nut onto it.