When Do I Use a Long or Short Trade?

You would go long or use a long trade on a stock that you believe or know will rise in price. A long trade to a day trader is, at most, one trading day. If you find an opportunity to enter a trade, and you know the stock price will increase (and be desirable for another trader after you buy it), you'd go long on that stock.
You would go short on a trade if you know the price was going to decline. Your broker must borrow the shares from the owner (probably another broker) or lend them to you if they own them. If the broker can't borrow the shares for you, you're not going to be able to short the stock. Stocks that just started trading on the exchange—called Initial Public Offering stocks (IPOs)—are not shortable (able to be sold then bought).
Traders can go short in most financial markets. A trader can always go short in the futures and forex markets (different from the stock market). Most stocks are shortable in the stock market as well, but not all of them.
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