Extension:

To understand this, you must first make clear what is the substitution effect and what is the income effect. Substitution effect refers to the change of relative price caused by the change of commodity price, which leads to the change of consumers' demand for commodities at the same utility level, so substitution effect is a change of demand. Therefore, whether for normal products, inferior products or Ji Fen products, the corresponding price drop demand is increasing, so it is a reverse change.

Income effect refers to the change of consumers' actual income due to the price change of a commodity, which leads to the change of demand. It can be seen that the income effect is also the change of demand, so for normal products, the actual income will increase when the price drops, and the demand will also increase, so the price and income effect will change in the opposite direction, while for inferior products and Ji Fen products, the price drop and the actual income increase will lead to the decrease of their purchases, that is, the demand will decrease, so the income effect will change in the same direction as the price.

As for the relationship between aggregate effect and price change direction, we need to compare the influence of substitution effect and income effect on demand. For reference.