Complex sentence: The artist planned to marry Elizabeth Jane Gardner, a pupil whom he had known for ten years, but his mother was opposed to the idea so soon after Nelly's death, and made William swear he would not remarry in her lifetime.
Split into simpler sentences: The artist planned to marry Elizabeth Jane Gardner, a pupil whom he had known for ten years, but his mother was opposed to the idea. Soon after Nelly's death, she made William swear he would not remarry within her lifetime.