Complex sentence: He started work on it in 1815, and completed it on the morning of the premiere of his opera "Der Freischütz", 18 June 1821, and he premiered it a week later, on 25 June, at his farewell Berlin concert.
Split into simpler sentences: He started work on it in 1815, and completed it on the morning of the premiere of his opera "Der Freischütz", 18 June 1821. He premiered it a week later, on 25 June, at his farewell Berlin concert.