This paper focuses on the teaching of en to non-native speakers of French. We show that the substitution rules generally listed in textbooks to teach this pronoun are problematic because they do not describe crucial facts. For instance, constituents introduced by des (e.g. des biscuits ‘(some) biscuits') and de + bare noun expressions (e.g. pas/peu de pain ‘no/little bread'), very frequent in every day French, are not covered. To solve the problem, we propose that the concept of reference ((in)definite/partitive reference) be explained to the learner. More generally, we point out that substitution rules are limited because they only take into account morphological aspects but not the context/discourse, where reference is accessible.