We propose a scheme to generate single-cycle terahertz (THz) pulses via reflection of an optical femtosecond pulse train from a thin layer of nonlinear resonant medium. Our method is based on the coherent control of low-frequency oscillations and free induction decay in the medium. The specific single-cycle shape of generated THz pulses requires a plane wave front and detection in the near field. Our theoretical results pave the way to a new, simple, and highly efficient way to generate single-cycle waveshape-tunable THz pulses.