Partial differentiation means finding how one variable changes with respect to another one variable, with no other variables making any difference.

So in this case, if you were finding , that would mean you are asked to describe how changes from changes in . Note that will not have any effect, and so gets treated as constant.

If you treat as being constant, then

(because the derivative of the constant is 0).

The only way you could find is if you knew that was some function of . If it was, then

.