Complex sentence: It can be open (with negative spatial curvature), flat, (positive spatial curvature), although if it is closed, sufficient dark energy must be present to counteract the gravitational universes will expand forever even in the absence of dark energy.
Split into simpler sentences: It can be open (with negative spatial curvature), flat, (positive spatial curvature), although if it is closed, sufficient dark energy must be present to counteract the gravitational forces. Open and flat universes will expand forever even in the absence of dark energy.