Complex sentence: In 1979, the 79 was to be replaced by the Lotus 80, intended to be the next step in the evolution of ground effects, but the 80 proved a failure, and modified Lotus 79's were used, driven by Andretti and Carlos Reutemann.
Split into simpler sentences: In 1979, the 79 was to be replaced by the Lotus 80, intended to be the next step in the evolution of ground effects. But the 80 proved a failure and Lotus was forced to go back to the 79, driven by Andretti and Carlos Reutemann.