Substitute is temporary. Replacement is permanent.

exactly. For example, If a teacher gets sick and can't come to school then she gets a substitute teacher for a day or two. If a teacher is pregnant and takes maternity leave for the rest of the year to have and raise her baby then she needs a replacement teacher.

If a part of my car breaks down I might need a new part for it. The garage would fit a replacement part. However, if the car is old and the correct part is no longer available they would fit a substitute part. In this example, the substitute part is not the same as the one used previously but is an alternative.