Why is it so much harder to walk in sand as apposed to a hard surface?

Because part of the energy expended in taking a step is used to push the sand away from its original position; a motion and use of energy that does not advance the walker, and is essentially wasted.

Good Question TV.

According to what I know, the harder the surface_the more the friction. From Newton's 3rd law we understand that:
More friction means we'll receive more forward thrust. Meaning, we'll get a push-forward while walking on a harder surface.

Whereas in sand there are two problems.
1) Sand is softer (so the lesser friction, lesser thrust thingee...)
2)Sand is so soft that instead of our foot being pushed forward, due to our weight, it gets pushed down into the sand!

Hope I'm able to solve your query,
Pin~Jinx

Think of it like billiard balls. If you roll a billiard ball at a wall, the wall doesn't move significantly and the ball bounces back just as fast as it was moving towards the wall. if you roll a ball at another ball, however, the first ball doesn't roll back very quickly. a lot of it's momentum was lost to moving the second ball. in the same way, a lot of your momentum is lost pushing the sand around.