In this Chess Lesson the beginner (Black) gets overrun in the center by the expert (White). This happens because Black recaptures with the bishop in move 8, but should recapture with the pawn instead to stop the white pawn storm starting with 9.f4!.
Black has to retreat his bishop and runs behind in development time. He is unable to get his knight out because it has no proper squares. The knight gets chased back to its original square and Black cannot complete his development and is unable to castle. This paralyzes his whole setup and finally White gets a strong winning attack that is unstoppable.