Complex sentence: They began collaborating on songwriting, with King writing the music and Goffin the lyrics, and began a relationship; after King became pregnant, they married in 1959 when he was 20 and she was 17.
Split into simpler sentences: They began collaborating on songwriting, with King writing the music and Goffin the lyrics, and began a relationship. When King became pregnant, they married in 1959 when he was 20 and she was 17.