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.
Because your legs keep moving
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.
Maybe, maybe not. Forces don't happen because you're moving ... forces are what cause you to move. If there are no forces acting on you, then you keep moving whichever way you're moving, and your speed or direction don't change. If there are forces acting on you, then your speed and/or direction can change. If the force is in the same direction that you're already moving, you'll move faster (accelerate). If the force is in the direction opposite to the direction you're moving, then your speed will decrease. Does any of this sound surprising ???
Gravity and velocity (or inertia), and they aren't things. They're forces.
The two things that keep the Earth in its orbit around the Sun are the gravitational pull of the Sun, which creates a centripetal force that keeps the Earth moving in a curved path, and the inertia of the Earth, which causes it to continue moving in a straight line unless acted upon by an external force. Together, these two forces balance each other to maintain the Earth's orbit.