Complex sentence: Keats and Severn had already known one another in England, but they were only passing acquaintances, yet it was Severn who agreed to accompany him to Rome when all others could, or would, not.
Split into simpler sentences: Keats and Severn had known one another in England, but they were only passing acquaintances. Yet it was Severn who agreed to accompany the poet to Rome when all others could, or would, not.