well there are three basic reason for downward slope of demand curve.First is the income effect.When price drop or fall people can buy more or less of a good for the same amount of money.Second is the substitution effect.If consumer dont see meaningful difference between products they'll buy the one with lowest price so a price increase will drive them toward substitutes while a reduction will draw them in.Third is the concept of diminishing marginal utility. If u already have plenty of something u have less of a need to buy more of it.now lets understand downward sloping of demand curve with the help of a table:-price quantity demanded 5 14 2 3 32 4 1 5 This table clearly shows that more and more units of commodity are demanded when price of the commodity falls.When u draw this table u ll find that the demand curve slopes downward from left to right indicating an inverse relationship between price and quantity demanded.