How does potential and kinetic energy change in to each other and back again?

They can change because the law of conservation of energy allows it to happen, for example when you are sitting, your body is at a potential energy state, meaning you are inert, you are not moving, but when you get up and suddenly start walking or running, that energy is converted to kinetic energy, meaning that you are moving and can be changed back into potential energy if all of a sudden you stop running or walking to rest or sit down. This is just an example of how energy can are transferred multiple ways

Pewnej nocy zakradł się pod pobliski dwór i, gdy nikogo nie było w kredensie, sięgnął po wiszące na ścianie skrzypki. zrobił to zbyt nieostrożnie i zbudził lokaja, który oskarżył janka o próbę kradzieży. sąd, nie chcąc posłać go do więzienia, kazał stójce stachowi zbić go rózgą. chłopak, zbity bardzo mocno, zmarł trzeciego dnia.

Efficiency is an action of doing something. for example when you take a test. good efficiency means you did well but bad effeciency or no effiency means you did bad.