Complex sentence: The 1951 Grand Prix motorcycle racing season was the 3rd F.I.M. Road Racing World Championship season, with eight Grands Prix and five classes, 500cc, 350cc, 250cc, 125cc and Sidecars 500cc.
Split into simpler sentences: The 1951 Grand Prix motorcycle racing season was the 3rd F.I.M. Road Racing World Championship season. The season consisted of eight Grand Prix races in five classes 500cc, 350cc, 250cc, 125cc and Sidecars 500cc.