Complex sentence: The design was not proceeded with until results from trials of HMS "Nautilus" and "Swordfish" had taken place, and in the event a slightly smaller three-screw submarine, the J class, was designed.
Split into simpler sentences: The design was to not proceeded until results from trials of HMS "Nautilus" and "Swordfish" had taken place. After the trials, the slightly smaller three-screw submarine, the J class, was designed.