Why are our feet fixed when walking? Why are our feet fixed when walking? Why does the static frictional force acting on our feet not push our feet forward ?

 A: If the foot is not accelerating, then that means it has a net force of $0$ acting on it. Since the static friction force acts forward, this means that there is another force acting on the foot that has a component pointing backwards. The only place this force can come from is the leg, and this makes sense. By Newton's third law, if the leg is pushing backwards on the foot$^*$, then the foot is pushing forwards on the leg, which is what we want to have happen if we are trying to walk forward.
Of course this is a huge over simplification of walking, which involves multiple steps (pun always intended) and many interconnected body parts, forces, etc. But I think just the simplistic view I present above is useful in the initial understanding of why the net horizontal force acting on the foot is $0$ for the part of the step where the foot is on the ground.

$^*$Of course vertical components of these forces are important as well, but of course the vertical components of the forces acting on the foot cancel as well (at least in the part of the step where the foot is not accelerating vertically). I don't think analysis of these vertical components is important for this discussion focusing on horizontal movement due to friction though.
A: Because the static friction is just enough to balance the force applied by us. However there is a maximum value to it. Also if its going to make our feet move forward, then kinetic friction will be generated, the friction won't be static in that case. 
A: Friction cannot accelerate an object, only slow it down. This is due to the second law of thermodynamics. The frictional force from the floor and the force exerted on the foot by the leg balance out and the foot does not acclerate while in contact with the floor.
A: Friction is a force that opposes the relative motion of bodies in contact.when we are stand on our feet ,there is no force that make relative motion So there is no Friction. 

If some how there is a force that act on feet somehow the static friction just balance it,it can't be greater than that force.
1:For bodies not in relative motion (static friction),
$$0 \leq f\leq \mu N$$.
