By definition spoken dialogue CALL systems should be easy to use and understand. However, interaction in this context is often far from unhindered. In this paper we introduce a formative feedback mechanism in our CALL system, which can monitor interaction, report errors and provide advice and suggestions to users. The distinctive feature of this mechanism is the ability to combine information from different sources and decide on the most pertinent feedback, which can also be adapted in terms of phrasing, style and language. We conducted experiments at three secondary schools in German-speaking Switzerland and the obtained results suggest that our feedback mechanism helps students during interaction and contributes as a motivating factor.