Supply and demand are the basis of economics and it is important to identify their patterns and graphs. There are three main reasons for demand:

1. The income effect: Customers will buy more products when prices fall and the opposite if prices rise

2. The substitution effect: If the price of a substitute is lower than other similar products, then more of it will be purchased. 

3. Diminishing marginal utility: The more a good one consumes, the less utility and happiness one will get from it. 

A change in quantity demanded is when a price and demand change to any place along the existing demand curve.

A change in demand is when the entire demand curve shifts as shown in this figure. This is often due to the determinants of demand which are:

S - Substitutes are goods that share a similar market
P  -  Preference  is a change in the consumer's preferences and the population is the number of customers in a market
I  -  Income  When the income increases the demand goes up
C -  Complementary goods are goods that must be used together
E - Expectations that prices may increase or decrease

The supply curve increases when prices increase as sellers are more potential consumers. The determinants of supply are as follows:

R - Resource costs are when the price of producing a product changes
O - Other goods' prices  determine what types of goods a firm will produce
T - Technology  can increase efficiency and lower costs
T - Taxes are mandatory payments to the government 
E - Expectations of price increase will encourage suppliers to produce
N - Number of sellers changes the amount of competition a market will face