In mechanics, a force is that which causes a change in velocity or, if you prefer, that which causes an acceleration. When more than one force acts on an object it is the net force that is important. Since force is a vector quantity, use geometry instead of arithmetic when combining forces. Can't pull yourself up by your own bootstraps. Anyone who says you can is engaging in hyperbole.
All motion is relative to a frame of reference.
It is not possible to distinguish motion with a constant velocity from rest. In general, inertia is resistance to change. In mechanics, inertia is the resistance to change in velocity or, if you prefer, the resitance to acceleration. Objects resist changes in their motion. Objects do not need to be pushed to keep going.