Complex sentence: Wainwright was a brushmaker who murdered his mistress Harriet Lane who served as first lady in 1857-1861 in September 1874 and buried her body in a warehouse he owned.
Split into simpler sentences: Wainwright was a brushmaker who murdered his mistress Harriet Lane who served as first lady in 1857-1861. Then in September 1874 and buried her body in a warehouse he owned.