Short answers, pulled from the story.
Edward III declared war on Philip VI in 1337 because the French king moved to confiscate the Duchy of Aquitaine, specifically the province of Gascony, which had been a source of tension since Edward III paid homage to Philip VI in 1329. This feudal dispute transformed into a dynastic war when Edward III revived his claim to the French throne based on his bloodline as the son of Charles IV's sister Isabella.
The Black Death arrived in Europe between 1347 and 1351 and killed between 30 and 60 percent of the French population, amounting to 6 to 12 million deaths, while England lost roughly 2 to 3 million people. This biological catastrophe caused economic collapse, labor shortages, and social dislocation, which weakened royal authority in both kingdoms and frequently interrupted the war with truces.
The Battle of Agincourt in 1415 resulted in the deaths of many French nobles, with about 40 percent of the French nobility killed, and secured English control over much of northern France. This stunning victory allowed Henry V to formalize the English claim through the Treaty of Troyes in 1420, which declared the Dauphin Charles VII illegitimate and named Henry V as the heir to the French throne.
Joan of Arc was burned at the stake on the 30th of May 1431 after being captured by the Burgundians at the siege of Compiègne in 1430 and convicted of heresy by Pierre Cauchon, the Bishop of Beauvais. Her martyrdom strengthened French resolve and marked a turning point in the war as the French began to gain the upper hand thanks to the leadership of Charles VII and the development of a professional standing army.
The Hundred Years War ended with the French victory at the Battle of Castillon in 1453, which was the first major battle won through the extensive use of field artillery led by Jean Bureau. The English were decisively defeated and lost all continental possessions except Calais, which contributed to internal unrest and financial difficulties that led to the outbreak of the Wars of the Roses in 1455.