Complex sentence: Warren Buffett played a significant role in growing the endowment at Grinnell, where he served as a trustee from 1968 until 2011, when he decided to step down from all boards not relating to Berkshire Hathaway.
Split into simpler sentences: Warren Buffett played a significant role in growing the endowment at Grinnell, where he served as a trustee from 1968 until 2011. In 2011 he decided to step down from all boards not relating to Berkshire Hathaway.