Complex sentence: Utterson goes out walking with Enfield, and they see Jekyll at a window of his laboratory; the three men begin to converse, but a look of horror suddenly comes over Jekyll's face, and he slams the window and disappears.
Split into simpler sentences: Utterson goes out walking with Enfield, and they see Jekyll at a window of his laboratory. The three men start conversing, but a look of horror suddenly comes over Jekyll's face, and he slams the window and disappears.