Stage Two is where we continue to have a high birth rate but the death rate starts to fall and therefore the population expands and you get population growth
Stage Three is where the death rate is low and your birth rate starts to fall, but your population is still rising because your birth rate is still higher than the death rate. We have entered this stage now. In the 1800s the death rates fell significantly due to the improvements in health, medicine and sanitation.
Stage Four low fluctuation where you have both a low birth rate and a low death rate. Therefore, they cancel each other out but the population remains high. This