A simple phenomenological model with two coupled structural and ferroelectric order parameters is under consideration. The small-signal dielectric response, dielectric response to a strong electric field (hysteresis phenomena included), and the phase diagram in the presence of the bias field are analyzed in the framework of Landau theory. It is shown that for the set of properties studied one can single out six qualitatively different types of behavior of the model. It is also shown that, in the framework of a unified phenomenological scheme, this model can describe the gradual conversion of ferroelectric to antiferroelectric behavior. The application of the model to the description of real compounds with a sequence of phase transitions is discussed.