Complex sentence: "To Autumn" employs poetical techniques which Keats had perfected in the five poems written in the Spring of the same year but departs from them in some aspects, dispensing with the narrator and dealing with more concrete concepts.
Split into simpler sentences: To Autumn employs poetical techniques which Keats perfected in the five poems which he had written in the Spring of the same year. However Keats departs from some aspects of the previous poems, dispensing with the narrator and dealing with more concrete concepts.