We study several problems related to the construction and the use of effective lagrangians by considering an extension of the standard model that includes a heavy scalar singlet coupled to the leptonic doublet. Starting from the full renormalizable model, we build an effective field theory by integrating out the heavy scalar. A local effective lagrangian (up to operators of dimension six) is obtained by expanding the one-loop effective action in inverse powers of the heavy mass. Using this simple example we study the renormalization of effective lagrangians in general and discuss how they can be used to bound new physics. We also discuss the effective lagrangian after spontaneous symmetry breaking, and the use of the standard model classical equations of motion to rewrite it in different forms. The final effective lagrangian in the physical basis is well suited to the study of the phenomenology of the model, which we comment on briefly. Finally, as an example of the use of our effective field theory, we consider the leptonic flavour-changing decay of the Z-boson in the effective theory and compare the results obtained with the full model calculation.