How do you explain the HARDY WEINBERG principle in a lay man's language?

Under the conditions - 1. A large infinite population,2. Random mating takes place.3. No migration/immigration4. No mutations If the above conditions are present in a population, ratio of various genes in gene pool will remain constant. As we know, evolution is outcome of change in ratio of genes (natural selection), if gene ratio is not changing, no evolution will take place. We can also say that evolution will take place when the conditions in1 are not maintained, as is the case in nature. Thus, evolution takes place because- 1. No population is infinite large.2. There is always preference in mating and each member of a population does not have equal chance of mating.3. migration/immigration takes place in all the populations.4. Recombinations and mutations are always present. You can mathematically prove this law using simple equation- (a+b)2 = a2 + ab + b2 + ab where a and b are two genes. You can raise the power to3,4,5,6 etc(it denotes the generation) and you will find that ratio of a:b remains constant in all the generations.