Law of Demand states that there is a negative or inverse relationship between the price and quantity demanded of a commodity over a period of time.
Hence when we are determining demand's relationship to price, we keep other determinants of demand constant. These assumptions maintain the relationship constant:
The income of consumers remains constant.
No change in the size and composition of the population.
There are no changes in the price of substitute goods.
There are no changes in the taste and preferences of consumers.
No expectation of a price change in future.
There are no changes in the price of substitute goods.