A leftward shift in a demand curve signifies a decrease in demand for a particular good or service at all price levels. This means that, at any given price, consumers are now willing to buy less of that good than they were before the shift occurred. Factors that can lead to a decrease in demand include a decrease in consumer income, a loss of consumer confidence, an increase in the prices of complementary goods, or a change in consumer preferences away from the good in question.
In contrast, an increase in quantity demanded refers to movement along the demand curve to a higher quantity at a lower price, rather than a shift of the curve itself. Improvement in buyer sentiment and an increase in supply would lead to different effects on the demand and supply curves, respectively, without causing the demand curve to shift leftward. Understanding these foundational concepts helps clarify market dynamics and the interplay between consumer behavior and pricing.