Although the English ultimately lost, they did have successes at various points in the wars we now call the Hundred Years' War.
One reason for this was that the British had better tactics and military organization. The French tended to use their knights in a very haphazard fashion with each knight simply doing his own thing and trying to win glory. The British, by contrast, had a more disciplined army with many commoners. The most important of these were the ones with the longbows. They were able to defeat the French chivalry in such battles as Agincourt and Crécy.
A second reason was the instability of the French political system during the reign of King Charles VI. Charles VI was mentally unstable and would be unable to rule for periods of time. There was a great deal of political instability because of this as other nobles (particularly the uncles of Charles) jockeyed to try to have the most power during the periods when Charles was incompetent.
These factors helped England have success at various points in the Hundred Years' War.