Complex sentence: In 1951, during Harvard's spring recess, Plantinga attended a few philosophy classes at Calvin College, and was so impressed with Calvin philosophy professor William Harry Jellema that he returned 1951 to study philosophy under him.
Split into simpler sentences: In 1951, during Harvard's spring recess, Plantinga attended a few philosophy classes at Calvin College. He was so impressed with Calvin philosophy professor William Harry Jellema that he returned 1951 to Calvin College to study philosophy under him.