To answer your question, time only moves in one direction from our point of view because we lack the ability to move through it freely. If you think about it from a 2 dimensional perspective, if a 3d sphere passed through the 2d plane, a 2d person would only see a circle growing bigger until stopping and start shrinking once again. This would be the cross-section of the sphere moving through it. The 2d plane would be “moving” through the height of the sphere just as we “move” through time/timespace. The 4th dimension is classified as “timespace” because it is a way to classify your position in not only physical 3 dimensional space, but time as well. I would definitely recommend looking up articles on something called “super-string theory.” It explains a theory that there is 10 dimensions, each with its own “pieces of the universal puzzle” if you will.