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The acquisition of /E/ verbal endings represents a real hurdle for the pupils: eight frequent written forms arise for a single phonological form ( trouvé, trouvait, trouver, etc.). The aim of this article is to present various data which illustrate the answers and the knowledge of the students faced with this difficulty. These data were collected during experiments carried out in school with a range of populations from 8 to 15 year old pupils (two cross-studies and two longitudinal ones). We present the main tendencies of the listed errors that reveal the induced and sometimes contradictory constraints inferred by the orthographical system. We organize them according to three axes. First of all, we examine if the selections implemented by the pupils are influenced by statistical or category-specific constraints. The conditions of realization of the agreement are then examined to determine if the selections conform with the standard ( ils ont trouvé) or not ( ils trouvés / ils ont trouvés). The selection of plural marks is approached in term of the candidates (? s vs ? nt). We also focus on the case of surgeneralisation of the agreement, a tendency which increases with the age and which one observes in two differentiated school contexts, in France and Quebec. The interactions between constraint of agreement and selection of the morphonogram are finally approached.