Principle of capacitor

You have to think in terms of potential . Consider the first plate, say $A$. It can be charged to a maximum value $+Q$ since any further increase in charge will cause a leakage of charge due to the increase in potential. You can imagine that. The potential of a charge distribution decreases slowly than the electric field with distance. So we need no leakage of charge. Now, we place another plate, say $B$, in the vicinity of $A$. The face of $B$ close to $A$ will earn a negative charge same as the positive charge in magnitude. So that face earns a negative charge $-Q$. by induction. This $-Q$ charge on one of the faces of $B$ induce a positive charge of same magnitude as that of negative charge on the other face of $B$. Hence the other face of $B$ earns a charge $+Q$. The negative charge on $B$ try to decrease the potential of $A$. At the same time, the positive charge on $B$ try to increase the potential of A. But the effect of the negative charge on the closer face of $B$ w.r.t. $A$ will be higher. So the potential of A reduces.

Now we ground B. The excess positive charge on $B$ flows to the ground and on that face, develops a negative charge. This more negative charge enables $A$ to hold more positive charge. this is the principle of a capacitor. Since the charge on the pate increased, the effective capacity to store charges in the plate is increased. Also the electric field is increased.

You can also see that the electric force between the two plates is attractive. So the potential energy of the charge configuration should decrease as attractive forces are more stable. Dielectric break down means a long reduction in the resistance of an electrical insulator when the voltage applied across it exceeds the breakdown voltage. This results in the insulator becoming electrically conductive. However, in this configuration, the voltage between the plates is reduced. So there is no chance of dielectric breakdown.