How did the Europeans change the institution of slavery in africa ?

Although slavery had long been practiced in Africa before the arrival of the European slave trade, slavery in Africa by Africans had been mostly imposed on the losers of wars and territorial skirmishes, or to settle individual debts or disagreements between warring tribes. Slavery had not been widely used for strictly economic purposes, however. This all changed with the arrival of the European slavers, who used the indigenous slave traders, mostly from West and North Africa, to kidnap men, women and children from other territories and sell them into bondage for the long, often deadly voyage to colonies in the West. 

Prior to the arrival of the European slave traders, slavery in Africa had not necessarily been a permanent state. As mentioned above, it was often a punishment or a way of settling a score with a rival tribe. Often it involved sexual slavery, with women bearing the brunt of the effects. It was certainly less systematic and organized than it would become with increased demand from British, French and Portuguese Colonies. 

Once the slave trade in Africa became part of the global Triangle Trade, African Slaves became a commodity and Africans who lived near the coasts started to see and hear about the fate that befell others who were sold into bondage. Many of those who hadn't already been kidnapped and sold into slavery fled from parts of the coasts where it was known that the brutal Slave Traders were active. This in turn pushed the African Slave Traders to go deeper into the continent to get procure more slaves for the increasing demand of plantation owners in the American Colonies, the Caribbean, and places like Brazil. The greater the demand grew, the more profitable the trade became and the more ruthless the methods of the African slave traders became. This vicious cycle tore apart the fabric of many African societies, and gutted many parts of the continent's most able-bodied populations.