Complex sentence: In Canada's constitutional system, one of the main duties of the Crown is to appoint a prime minister, who thereafter heads the Cabinet and advises the lieutenant governor on how to execute his or her executive powers over all aspects of government operations.
Split into simpler sentences: In Canada's constitutional system, one of the main duties of the Crown is to appoint a premier. Once the premier is installed, he or she thereafter heads the Cabinet and advises the lieutenant governor on how to execute his or her executive powers over all aspects of government operations.