When does a body accelerate?

A body accelerates when force acts on it. Actually, we should say a "net force" ... if there is more than one force acting on the body, they may have different strengths and come from different directions. Then, we have to combine their strengths and directions to find a single force that's equivalent to all the separate ones. If the strengths and directions are just right, it's possible that they can all cancel each other out ... for example, if there are two forces on the body, with equal strength and exactly opposite directions, they add up to zero, just as if there was no force at all acting on the body. So we should actually say that a body accelerates when the net force on it is not zero.