Complex sentence: Juan Carlos became King on 22 November 1975, two days after Franco's death, the first reigning monarch since 1931; although his exiled father did not officially abdicate in favor of his son until 1977.
Split into simpler sentences: Juan Carlos became King on 22 November 1975, two days after Franco's death, the first reigning monarch since 1931. His father did not abdicate in favor of his son until 1977.