We focus on a subdiffusion–reaction system in which substances are separated at theinitial moment. This system is described by nonlinear differential subdiffusion–reactionequations with a fractional time derivative. These equations are very difficult to solvebut there exist methods which allow us to solve them approximately. We discuss how usefulsuch methods are, in particular, the quasistatic approximation method and the perturbationmethod in analytical solving subdiffusion–reaction equations.