Demand
The quantity of a good consumers are willing and able to buy at each price, holding other factors constant.
The classic item gives a scenario and asks whether quantity demanded or demand changed — the tell is the wording. Own-price moves you along the curve; any non-price determinant (income, tastes, expectations, prices of substitutes/complements, number of buyers) shifts the whole curve, so map each driver before answering. Vignettes that bury a substitute’s price change or an income shift are testing exactly this trap. Watch the algebra too: in the demand function Qd = a − b·P the own-price coefficient is negative; the curve you graph (price on the vertical axis) is the inverse demand function, with price written as a function of quantity. A genuinely positive own-price coefficient is the anomaly — a Giffen/Veblen good — not just inverse notation.
Don’t conflate the demand curve with the upward-sloping supply curve. Equilibrium sits where the two cross, so price is set jointly — demand alone never fixes it. A frequent error is calling a luxury “inferior” — inferior goods have demand falling as income rises (a leftward shift), the reverse of normal goods. Memory hook: own price = along, anything else = shift.
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