Substitutable items, such as apples and oranges, can be substituted for each other (provided that you love fruit and don't mind eating apples or oranges). If the price of apples falls, so will your demand for oranges. Unless you don't care about the price.

Is the stock upstairs right or wrong? From the investment point of view, because there are many things that can be invested, such as real estate, various futures, options, etc., if the stock market has an obvious downward trend or crashes, you will want to invest in other places instead of the stock market, so the demand for stocks has decreased, which is the result of mutual substitution.

It is said that he is wrong, because stock traders can use something called short selling to sell stocks at the current price when they are expected to depreciate. You are selling someone else's stock, because you have nothing at hand now. Wait until it falls, then buy it back at that price and return the stock you borrowed. You earned the difference. If you want to lend you someone's stock, post it when you need an answer, because explaining it clearly is more complicated than explaining it.