Complex sentence: As a Ontario CCF MPP in the provincial legislature from 1943 until 1951 Dennison was an early environmentalist and, in the 1940s, tried to stop the de Havilland aircraft factory's pollution of the Black Creek River.
Split into simpler sentences: As a Ontario CCF Member of Provincial Parliament (MPP) in the Legislative Assembly of Ontario from 1943 until 1951, Dennison was an early environmentalist. In the 1940s, he tried to stop the de Havilland aircraft factory's pollution of the Black Creek River.