Complex sentence: Its main base is Cologne Bonn Airport, with secondary bases at Stuttgart Airport, Hannover-Langenhagen Airport, Berlin-Schönefeld International Airport, Hamburg Airport and Dortmund Airport.
Split into simpler sentences: Its main base is Cologne Bonn Airport, with a secondary base at Stuttgart Airport. Other bases are Hannover-Langenhagen Airport, Berlin-Schönefeld International Airport, Hamburg Airport and Dortmund Airport.