Whenever another force - usually friction - slows it down.
Things would were it not for opposing forces such as friction.
gravity
That's true. Plus ... even if the forces on an object are BALANCED, it can keep moving forever.
As long as the forces remain the same or do not change direction
The force that prevents two forces in contact from sliding past each other is friction. Friction has the ability to keep things from moving.
If an object is moving, no force is required to keep it moving. Forces are required to CHANGE its motion, for example, to speed it up, or to slow it down.
Because your legs keep moving
No force makes the Earth move. It has enough momentum to continue moving through space.
No resultant forces act upon it
An object has a general tendency to keep moving - that's how our Universe works. No force is required to keep an object moving - unless there is another force that slows it down. Here on Earth, there are usually frictional forces that slow objects down, and therefore a force is required to counteract the frictional forces.
Because of something called inertia. Things that are moving likes to keep moving, and things that aren't moving likes to stay put. For anything to change from moving to stationary, or stationary to moving, there has to be an outside force acting on them.
If forces on an object are unbalanced, the object will accelerate or decelerate. If forces on an object are balanced, the object will stay still or keep moving with the same velocity.