We present a phenomenological picture for the many-body excitations of a two-dimensional electron gas in a quantum well spin polarized by a dc magnetic field at an angle ␪ with the axis of the well. In the framework of the Landau theory of charged Fermi liquids, we determine the frequencies of collective modes by solving a transport equation for quasiparticles in the local electromagnetic field associated with the charge- and spin-density fluctuations. In the long-wavelength limit, analytic solutions for ␻(qជ) are obtained as functions of the degree of spin polarization and of the angle ␪.