The generalized hydrodynamics equations, including nonlocal behavior in time, are used to calculate density response functions for impulsive stimulated light scattering (ISS) from glass-forming liquids. Single-exponential relaxation dynamics are considered first, and then the results are generalized to account for complex relaxation. Thermal diffusion, acoustic, and relaxation modes are described. The time dependence of both acoustic and relaxation modes yields the structural relaxation dynamics. In addition, the relative amplitudes of the modes in ISS data yield the Debye-Waller factor f(q), called the nonergodicity parameter in mode-coupling theory (MCT) of glass-forming liquids, in the limit of low wave vector q. This permits testing of MCT predictions of a square-root cusp in f(q-->0)(T) at a crossover temperature T-c. The information which can be obtained through ISS is compared in theoretical and practical terms to that obtainable through low-frequency light scattering spectroscopy.