Complex sentence: Today branches of the family currently reign in Belgium, the United Kingdom and the Commonwealth of Nations and the former Tsar of Bulgaria, Simeon II, kept his surname as the Prime Minister of Bulgaria from 2001 to 2005.
Split into simpler sentences: Today branches of the family currently reign in Belgium, the United Kingdom and the other Commonwealth realms. The former Tsar of Bulgaria, Simeon II, kept his surname as the Prime Minister of Bulgaria from 2001 to 2005.