Complex sentence: In his rookie major league season in, Jansen won 21 of 26 decisions, leading the National League in winning percentage (.808) and finished second in the voting for Rookie of the Year behind the Dodger's Jackie Robinson.
Split into simpler sentences: In his rookie major league season in, Jansen won 21 of 26 decisions, leading the National League in winning percentage (.808). He finished second in the voting for Rookie of the Year behind the Dodger's Jackie Robinson.