why does a thunderstorm occur always before a mosnoon and after the monsoon ends?

Answer 1

because of all the movements in the clouds build up static electricity and then it causes lightening...

Answer 2

Usually a cold front (low pressure system) is invading a warm front (high pressure system). Before the "monsoon". The two systems collide which causes energy. The release of energy is the beginnings of lightning (thunderstorm).Then the rains come in afterwords (monsoon). It is the reverse of this effect at the end of the monsoon.