We examine the impact of charge trapping on the memory window reduction in FeFETs. Different from nonferroelectric FETs, the nonlinearity of ferroelectric polarization results in nonlinear relation between the trapped charge density N_t and V_th shift. It is confirmed that the memory window cannot be estimated by the linear superposition of the V_th shift by polarization and by charge trapping. We also provide a critical discussion of some misunderstanding about N_t-V_th relation that causes an incorrect estimation of N_t in FeFETs.