Question

Why is frictional force 0 in the case of pure rolling on a plane surface and present in the case of pure rolling on an incline?

Answer 1

on a plane surface frictional force act on a body only if it has translatory motion,i.e. a force is parallel to the plane producing motion parallel to the plane but frictional force is zero for pure rolling on a plane surface.
on an incline the force of gravity parallel to the inclined plane affects the angular velocity of the body,i.e. affects its rolling motion.so the frictional force here comes into play opposing this force parallel to it.therefore frictional force is present in the case of pure rolling of an incline.