Complex sentence: Nxe5, White usually plays 5.d4 (5.f4 does nothing for his development), after which Black can retreat the attacked knight to either g6 or c6.
Split into simpler sentences: Nxe5, White usually plays 5.d4 (5.f4 does nothing for his development). After 5.d4, Black can retreat the attacked knight to g6 or c6.