Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...f5/4. d3/4...fxe4/5. dxe4

5...Nf6 is the only move. As well as attacking the e4-pawn, it's a case of making the easy decisions first: the g8-knight is the only Black piece that has an obvious square to go to.