Fixing Sink Stopper?

Question

Our sink stopper is a metal one and just recently it got stuck in the down position (that you put it in to fill the sink with water). The sink wasn`t filled with water and using a small screwdriver we got it out, but now we are having trouble putting it back in to work correctly. Everytime we put it back in, it sinks into the down position. Isn`t it supposed to hook to something inside? What`s the easiest way to fix it without removing/damaging pipes?

Answer 1

what tool do I use to take off the nut on the vertical pipe that is holding the metal stopper