This work characterized the "interlanguage" of students learning Czech as a foreign language. The description is based on a long-term research, analyzing errors in texts written by students whose mother tongue is not Slavic. The scholar examined mistakes in all linguistic areas, focusing particularly on morphology and syntax. Errors were described as they developed over the course of one academic year. A quantitative analysis presents the occurrence of mistakes in written texts produced by non-Czech speakers. A qualitative analysis of systematic mistakes allows for formulating a hypothesis on the causes of these errors and present proposals for the improvement of the methodology of teaching some language phenomena.