The major condition that led to the French Revolution was the fundamental inequality between the "estates."
Before the Revolution, French society was broken up into three estates -- the nobility, the clergy, and the commoners.
Of these three estates, the commoners were the poorest and the most numerous. Unfortunately, they were also the ones who had the least political power and they were the ones who paid all the taxes
So you had a situation in which the rich and powerful paid no taxes while the poor, numerous, and powerless commoners did pay.
This was the main cause of the Revolution.
French revolution, which lasted from 1789 to 1799, introduced the ideals of democracy in France and elsewhere but did not make France a democracy.
The direct trigger for the start of the french revolution was a financial crisis faced by the government. However there were many other underlying developments in political and economical conditions that led to the revolution. Major of these developments were dissatisfaction of lower and middle classes of people with the government, exposure to new ideas about equality, democracy and ways of governing, and financial problems brought about by excessive cost of wars.
At that time the French society was legally divided in three classes called estates. The first estate consisted of members of the clergy. Second estate consisted of nobles, and rest of the people wee included in the third estate. The third estates included peasants from the rural areas, working class from cities, and a large middle class made up pf professions such as merchants, government officials, ans lawyers.
Under the system that existed the people forming the third estate enjoyed very low social status and were discriminated against by the first and second estates, although the money for the luxurious lives led by people in upper classes were collected from the people in third estate. Economically the middle class people in the third estate were most important, and they resented their low social status.
At the same time liberal ideas about new ways of governing and challenging the powers of monarchies were popularised by some French writers and philosophers like Jean Jacques Rousseau. These writers emphasized that right to govern must be derived from the people.
When the discontent with the existing government was already very high, a financial crisis developed for the government due to various wars and the government was almost bankrupt in 1788. Under these circumstances, when the representatives of first and second state refused to give representatives of the third estate due representation in matters of approving new taxes, the representatives of third estate declared themselves the national assembly of France. This particular action can be treated as the starting point of French Revolution.