What must be true of the forces acting on objects if their motion is not changing?

If an object is moving in a straight line at constant speed, then we say it has no "acceleration".

This tells us that IF there are any forces acting on the object, then the forces must be 'balanced'.

That means that if you were to take the vector of each force and addum allup, their sum would be zero. THAT's why they have the same effect on the motion of the object as if there were NO forces at all acting on it.