Language errors made by non-native speakers are generated by various factors. From the lack of knowledge about specific grammatical rule, to its false understanding and individual and contextual mistakes that can be explained.
Another aspect is the way of categorisation of non-linguistic reality typical for the given output system. In the situation when L1 and L2 categorise the reality differently, the lack of knowledge about a certain rule causes strong and evident interference.
The paper analyses syntactic errors made by Czechs speaking Polish as their second language. Apart from the normative analysis, a cognitive interpretation of one of the discussed mistakes will be presented.