中四econ一問

Because change in demand is like change in attitude, so the demand curve itself is changed, this can be represented by a shift of the entire demand curve.
But change in quantity is only the quantity is being changed, not the attitude, so the demand curve itself is not changed, just have to lookup new values of supply & demand on the original demand curve.
A shift of the entire demand curve is moving the entire demand curve to right or left, to up or down, or even diagonal.
A movement along the same demand curve is use the original curve, input a new value to one axis, from the curve, read the corresponding value for the other axis.
I hope this can help with your understanding. =)