Fleming's left-hand rule shows how the force, the current and the magnetic field are arranged relative to each other. Hold the thumb, first finger and second finger of your left hand at right angles to one another. The thumb points in the direction of the force (the motion). The first finger points in the direction of the magnetic field, from north to south. The second finger points in the direction of the current, from positive to negative.
So the three directions are all at right angles to each other: thumb for force, first finger for field and second finger for current. If the current is reversed, or the field is reversed, the force is reversed. If both are reversed, the force is unchanged. If the wire is parallel to the field there is no force, so the rule is not needed.
