Complex sentence: Since the explosive season 4 finale, The crime duo have been dating till Castle popped the question in the season 5 finale when Kate has been offered a job in DC to work for the Attorney General's office.
Split into simpler sentences: Since the explosive season 4 finale, the crime duo have been dating. Castle proposed to Beckett in the season 5 finale when Kate had been offered a job in DC to work for the Attorney General's office.