  Using the principle of structural analogy of solutions, approaches have been
developed for constructing exact solutions of complex nonlinear PDEs, including
PDEs with delay, based on the use of special solutions to auxiliary simpler
related equations. It is shown that to obtain exact solutions of nonlinear
non-autonomous PDEs, the coefficients of which depend on time, it is possible
to use generalized and functional separable solutions of simpler autonomous
PDEs, the coefficients of which do not depend on time. Specific examples of
constructing exact solutions to nonlinear PDEs, the coefficients of which
depend arbitrarily on time, are considered. It has been discovered that
generalized and functional separable solutions of nonlinear PDEs with constant
delay can be used to construct exact solutions of more complex nonlinear PDEs
with variable delay of general form. A number of nonlinear reaction-diffusion
type PDEs with variable delay are described, which allow exact solutions with
generalized separation of variables.
