My Teacher is always writing that work done by external forces is equal to the negative of work done by conservative internal force and both of these are equal to change in potential energy. I don't understand it , can u please tell? Even I read in my book that we cannot change potential energy by doing work by external force.
It might be helpful to give an example of what your teacher is telling you.
Start with the work energy theorem which states that the net work done on an object equals its change in kinetic energy.
Take an object of mass $m$ initially at rest on the ground and raise it to rest a height $h$ above the ground. Since the object starts and ends at rest its change in kinetic energy is zero. From the work energy theorem that means the net work done on the object must be zero. Positive work of $mgh$ was done raising the object. For the net work to be zero, that means gravity, which is a conservative force the direction of which is opposite to the displacement of the object, had to do negative work of $-mgh$.
This what is meant by saying the work done by the external force (the one that lifted the object) it is the negative of the work done by the conservative internal force (gravity), or vice versa.
The end result is gravity took the energy given the object by the force that raised it, which is external to the earth-object system and stored that energy as gravitational potential energy of the earth-object system.
Hope this helps.