Compare perfect competition, monopolistic competition, oligopoly, and monopoly by power over price.
Markets differ by how much power sellers have over price. At one pole, perfect competition: many sellers, identical product (wheat is wheat) — nobody has pricing power; the market price is take-it-or-leave-it. At the other pole, monopoly: one seller, no substitutes — price is chosen, constrained only by buyers walking away entirely.
Most of life sits between. Monopolistic competition: many sellers, DIFFERENTIATED products (restaurants, sneakers) — branding buys a little pricing power. Oligopoly: a handful of giants (airlines, phone makers) watching each other like poker players, where one's price cut is everyone's problem. The competitive pressure a firm feels — and the consumer's deal — depends on which arena it's in. That's why societies police monopolies: competition is what keeps sellers serving buyers.
Mastery looks like: They classify real firms into the four structures and justify via substitutes and seller count.
Common stumbles: Calling any big company a monopoly; missing that differentiation (not size) creates pricing power.