By considering the surface plasmon mode in metallic grooves for true metals, we proposed a revised version of the analytical and numerical model on the light diffraction from a subwavelength slit surrounded by finite number grooves on a metallic film, originally published in Phys. Rev. Lett. 90, 167401 (2003). In comparison to numerical simulations by the finite difference time domain (FDTD) method, the revised model proves more accurate in describing the light diffraction behavior in the case of narrow grooves. A factor concerning the excitation efficiency of surface electromagnetic mode is proposed to measure the grooves' ability of tailoring diffraction patterns. Also discussed are the general characteristics of its dependence with geometrical structure parameters.