We show that in the standard model, when the mass of the top quark becomes very large, compared to other mass scales and external momenta, constraints develop in the field system. The top-quark field becomes nonlinearly realized. SU(2)U(1) remains a symmetry at the S-matrix level. A general formulation for the one-light-particle-irreducible Green's functional 1LPI and the effective Lagrangian Leff is presented in the setting of the external field technique and derivative expansion. The bosonic part of 1LPI and Leff is explicitly constructed. It encompasses all the top-quark effects for all low-energy bosonic processes. (Vertices with external fermions as well will be reported shortly.) Examples are given to show that our approach easily reproduces known results. Wess-Zumino terms due to the top quark are also given.