Are these assumptions correct? Is it true:From equation $C=Q/V$
1)If Potential difference is kept 
constant
$C \propto Q$
2)If charge is kept constant
$C \propto 1/V$
Then if 2nd is true I do have a question:
Don't we say that capacitance is the ability of a body to store charge,so my question goes this way,if the charge is kept constant and potential difference is decrease,then by the above assumption the capacitance should increase(and which is true).But we have said that charge is kept constant,then how would the capacitance increase?
I need an answer
 A: Well that's the same doubt people face when they say that heat dissipated is proportional to R (I*I*R) as well as inversely proportional to R (V*V/R)
My answer is that when you vary potential the charge WILL change unless you change the capacitance. If you have to keep the charge constant you have to vary the capacitance(i.e. separating parallel plates in case of parallel plate capacitor). By doing this you change the capacity of body to store charge.
And your equations are correct !
A: 2) would be antiproportional instead
A: Rather than saying that capacitance is the capacity to store charge you should consider it as the capacity to store energy. This lends itself well by writing the energy stored in a capacitor as: $U = \frac{1}{2} C V ^2$.
If you were to keep the charge constant on a capacitor, you can't have freely moving charges, so the capacitor wouldn't have a current running through it, this fixes the amount of charge. Then, by changing the geometry of the capacitor the potential difference across it would vary, which changes the capacitance.
