Why was the Fall of France a turning point in World War Two?
Typically, the Fall of France is not seen as a turning point. Turning points are those events like the Battle of Stalingrad where the Axis advance is stopped and the war turns in favor of the Allies. The Fall of France was not such an event.
If you have to argue that it was a turning point, you can say that it led to important events. Having now taken control of all of Western Europe, Hitler moved on to other targets that ended up causing problems for Germany. For example, the Fall of France led to the Battle of Britain which was a major setback for Germany. It also led to Hitler thinking that he could safely invade the USSR in June of 1941. This was, eventually, an even bigger setback for Germany.
So, the Fall of France can be seen as a turning point in that it allowed Hitler to set his sights on places like England and the Soviet Union. Attacking those countries helped lead to the defeat of Germany.