What role does static friction force play for a rolling object? How can I know what direction it points? Fundamentally, what does a friction force do for a rolling object? I am very confused by the way my textbook explains it. In my textbook , it says:
A wheel rolling on a horizontal flat surface at a constant velocity experiences no friction force. Why? 
On the same surface, there is an acceleration of the wheel pointing to the right (probably caused by a force), so the ball is angularly accelerating in the clockwise direction. In this case, a friction force appears, and it is also pointing to the right. How come? What does the friction do for this wheel?
On an inclined plane, a ball freely rolls down the surface. The direction of friction is up the ramp, which confuses me because in the previous example the friction force was in the direction of the wheel's acceleration. 
And there is a difference if a wheel is freely rolling and if there is a torque acting on the ball's center of mass. WHY???
What does friction do in these cases? How does it cause an object to roll? 
 A: *

*If the wheel is rolling at constant velocity without any other forces acting, then there is no tendency for slippage between the wheel and the surface, so there is no friction.  Friction will only act to try to prevent slippage between 2 surfaces.

*Presumably in this case there is a torque acting on the axis of the ball/wheel? This torque will act to give angular acceleration to the wheel (clockwise). If there was no friction then the angular velocity would increase and there would be slippage between the wheel and the surface. However, friction exists and will try to prevent the slippage, which is why it acts to the right. In trying to prevent this slippage, the friction also exerts a horizontal linear force on the wheel, giving it acceleration to the right, so it speeds up. This is how a car accelerates horizontally, using friction between the wheels and the road.

*On the inclined plane, the ball will tend to slide downwards and if there was no friction then again there would be slippage between the ball and the inclined surface. Perhaps the best way to think about it is: "Which way is slippage going to occur if there was no friction?" Friction will always try to counteract slippage, so here it acts up the slope to try to prevent the ball slipping down the slope. This has the effect of applying a moment to the ball, which will cause it to start rolling.
A: Hopefully this will answer at least some of your questions. If you consider a ball intially at rest on a frictionless surface, if a force is exerted through the centre of mass of the ball it will slide across the surface with no rotation, there will only be translational motion.
If you consider the case where there is friction, if the force is again applied to a stationary ball the frictional force will act in the opposite direction to the force but at the edge of the ball that rests on the ground. This friction applies a torque to the ball which starts the rotation. So static friction is infact necessary for a ball to begin rolling. 
The condition for rolling at speed $v$ is that the angular velocity of the ball is given by $$v=\omega r$$
from this the top of the ball will move at speed $2v$ the centre of mass of the ball will move at $v$ and the bottom edge of the ball will instantaneously be at rest. So as the edge touching the ground is stationary it experiences no friction.
So friction is necessary for a ball to start rolling but once the rolling condition has been met the ball experiences no friction. 
