This is the single most common "compare and contrast" pairing on the paper — knowing precisely what differs between the two runs, for each market structure, is worth more than any other single fact on this standard.
Short run vs long run, by market structure
Perfect competition
Monopoly
What can change in the short run
Price and output adjust; number of firms is FIXED
Price and output adjust; barriers still block entry
What can change in the long run
Firms enter (if profit) or exit (if loss) until profit = normal
Barriers persist, so economic profit CAN continue
Long-run profit outcome
Always normal profit only
Can remain supernormal, or fall to normal/subnormal if demand falls enough
The chain of reasoning for perfect competition specifically
Short-run supernormal profit → new firms are attracted by the profit opportunity → market supply rises → price falls.
This continues until price falls to the point where firms earn only normal profit — the signal to enter (or exit) disappears.
The reverse happens from a short-run loss: firms exit → market supply falls → price rises back toward normal profit.
✕
Assuming a monopoly always keeps earning the same profit long-term
A monopoly is shielded from ENTRY by barriers, but it is not shielded from a genuine fall in demand — if demand falls far enough, even a monopoly can slide from supernormal profit to normal profit or a loss. The barrier prevents new competitors, not a change in market conditions.
A fast check for a short-run vs long-run answer
Have I stated what triggers the change (entry, exit, or a shift in demand) explicitly?
Have I named the resulting profit TYPE, not just said "profit changes"?