In the five-character rhyme, the first sentence is flat and even. Why should the sentence be flat rather than flat?

Because metrical poems are mainly flat and parallel, the first sentence and the second, fourth, sixth and eighth sentences are flat and parallel, so if your example is "flat and parallel" and the last word is flat and parallel, it belongs to the law and is unqualified. Must be transferred to become ". It's called "the first rhyme of a flat tone"