Complex sentence: Runners may attempt to advance from base to base at any time (except when the ball is dead), but must attempt to advance when forced when all previous bases are occupied and a batted ball that touches the ground is a fair ball.
Split into simpler sentences: Runners may attempt to advance from base to base at any time (except when the ball is dead). A runner that must attempt to advance isforced, when all previous bases are occupied and a batted ball that touches the ground is a fair ball.