Complex sentence: The People's Republic of Bulgaria existed from 1946 to 1990 and it was ruled by the Bulgarian Communist Party (BCP), which in turn ruled together with its coalition partner, the Bulgarian Agrarian National Union.
Split into simpler sentences: The People's Republic of Bulgaria existed from 1946 to 1990. It was ruled by the Bulgarian Communist Party (BCP) ruled together with its coalition partner, the Bulgarian Agrarian National Union.