Complex sentence: Despite his youth, William Baines completed roughly 150 works, mostly in the genre of the piano miniature and he left a symphony, a poem for piano and orchestra and a number of chamber works.
Split into simpler sentences: Despite his youth, William Baines had completed roughly 150 works, mostly in the genre of the piano miniature. However, he also left a symphony, a Poem for piano and orchestra and a number of chamber works.