Final answer:
Substitute goods affect the demand curve by causing it to shift left or right, depending on their availability and pricing relative to the original product.
Step-by-step explanation:
Substitutes in consumption can affect the demand curve significantly. When a substitute product becomes more readily available or preferred, consumers might switch from the original product to the substitute, leading to a decrease in demand for the original product. The demand curve for the original product will shift to the left, indicating a reduction in the quantity demanded at each price point.
In the case where the price of a substitute good rises, the original product becomes relatively more attractive, and its demand may increase. In this scenario, the demand curve would shift to the right, showing an increase in quantity demanded at every price level. This shifting illustrates the principle that changes in the availability or pricing of substitute goods can lead to either an increase or decrease in demand for the concerned product.