This paper presents Diagram, a learning environment for object-oriented modelling (OOM) with UML class diagrams. Diagram an open environment, in which the teacher can add new exercises without constraints on the vocabulary or the size of the diagram. The interface includes methodological help, encourages self-correcting and self-monitoring, and provides the learner with specific graphic tools. Diagram includes a diagnosis module that compares the student diagram with a reference diagram. The differences that are noticed in the student diagram give rise to feedbacks. We illustrate the feedback elaboration process by giving an example of a diagram with the diagnosis results and the messages generated by the environment. We describe an experimentation in ecological context and analyze the effects of the feedbacks.