[Jai]

I am an issue understanding the changes that take place when we decrease the pressure of a reaction. I have understood that when we increase the pressure the side having more number of moles wants its moles to go to the other side where it is more empty(this is how I learnt it). But I fail to understand why if we decrease the pressure by increasing volume the side having less moles goes to the side having more moles? Also in the case of adding inert gases why does the equilibrium shift in the direction in which larger no of moles of gas are formed? Please help.

[ajax0604]

If the pressure decreases/volume increases, then the concentration of the particles decreases. The system tries to offset this change by shifting the position of the equilibrium to the side with more particles, thereby increasing the concentration. Remember though, that this only partially offsets the effect of the change. Adding inert gases won't shift the position of the equilibrium because they don't participate in the reaction.

[Jai]

 if we add inert gas at constant pressure. I think there should be 3 cases delta n>=or <0. My book says so.

[Ben Bob2]

if we add inert gas at constant pressure. I think there should be 3 cases delta n>=or <0. My book says so.

I'm unsure of what you're saying here, but maybe this quote will clarify things for you with regards to the inert gas:

Adding an inert gas into a gas-phase equilibrium at constant volume does not result in a shift. This is because the addition of a non-reactive gas does not change the partial pressures of the other gases in the container. While the total pressure of the system increases, the total pressure does not have any effect on the equilibrium constant.

Source: http://chemwiki.ucdavis.edu/Physical_Chemistry/Equilibria/Le_Chatelier's_Principle