  A method for constructing the low energy effective models for pairings in the
generalized Bernevig-Hughes-Zhang model for materials like Bi$_{2}$Se$_{3}$ is
proposed. Pairings in this two-orbital model are identified with those familiar
in one-orbital models, enabling a unified understanding. The theory provides an
easy way to understand the topological nature of the superconducting state that
is not directly related to the topological order in the normal state but due to
subtle coupling among the degrees of freedom. Furthermore this approach shows a
simple way to characterize the anisotropic nature of surface Andreev bound
states (SABSs). In particular, we have identified the conditions to have a
surprising new result of having two pairs of SABSs. It also leads to a
conclusion that SABSs always connect with the topological surface states if the
latter are well defined at the chemical potential.
