When price of a substitute for a goods falls then the demand for that goods will decline and when price of the substitute increases, the demand for that goods will increase. For example, when price of the tea as well as the incomes of the people remain the same but price of the coffee falls, the consumer would demand less of tea than before. Tea and coffee are very close substitutes, therefore when coffee becomes cheaper, the consumers substitute coffee for tea and as a result demand for tea declines.
