We learned in the class that monopoly markets do not have supply curve. Our textbook (Microeconomics and Behavior, page 388 from, R. Frank) says:
A monopolist has no supply curve
and it explains it by saying:
The reason is that the monopolist is not a price taker, which means there is no unique correspondence between price and marginal revenue when the market demand curve shifts. Thus, a given marginal revenue value for one demand curve can correspond to one price, while the same value of marginal revenue for second demand curve corresponds to a different price. As a result, it is possible to observe the monopolist producing $Q_1^*$ and selling at $P^*$ in one period, and then selling $Q_2^*$ at $P^*$ in another period.
I don't get this. We learned that in competitive markets demand can change from period to period even if the price stays the same (like when supply is just horizontal line). Also, why can't we just pick one point in time look at possible combinations between P and Q on monopoly market, and call those combinations supply? The book has some examples but they don't explain this clearly.