May I ask the origin of "Du Fu's three sentences, the first chapter of Nan Zhou" and how to understand it? Anxious to answer-

Nan Zhou's first chapter refers to "The Book of Songs", Nan Zhou's first poem "Guan Ju", and Du Fu's three sentences refer to the third sentence of Du Fu's four happiness poems: Wedding night. This is a traditional wedding couplet, which often appears on the doors of newlyweds.