The main sentence of "A Letter to Zhu Yuansi" was "The sunset clouds and the lonely ducks fly together in the autumn water, sharing the same color as the sky." It was not a sentence. The central sentence and the theme sentence are usually the same sentence, but in literary works, the term "theme sentence" or "central sentence" may sometimes be used to more accurately describe a sentence, which represents the core and theme of the entire work. A "theme sentence" is usually a short sentence that outlines the theme or theme of the work. A "central sentence" is the most representative sentence in the entire article that highlights the main idea or emotion of the work.