Theoretical insights are presented on multiple-wave mixing effects for a pump and a weaker probe beam incident on a nonlinear material, creating a refractive-index grating and diffractions. Diffractions up to second-order are included, and the role played by the diffractions in the forward amplification of the probe are taken into account. The coupled multiwave mixing effect has been solved in a self-consistent manner in widely varying conditions for all the relevant parameters. It is shown that due to the diffracted component alone the probe beam can experience gain as high as 20 in a 100-μm-thick liquid crystal or a 500-μm-thick silicon wafer. In the presence of a phase shift between the intensity grating and index grating, the amplification can be further enhanced or diminished. However, the interplay between the diffraction mediated amplification and the phase-shifted grating-induced two-wave mixing is not a simple additive process but rather is highly dependent on such parameters as pump beam intensity, self-phase modulation, and interaction lengths. The theory agrees well with results of experiments performed in several highly nonlinear materials.