The paper introduces the Czech learner corpus CzeSL as a source of relevant linguistic information about the errors of non-native speakers of Czech. It focuses on errors in the use of the relative pronoun který (who/which/that) and compares the scale of attested mistakes with the description of relative clauses in textbooks and grammars of Czech.
It reaches the conclusion that an analysis of corpus data may considerably contribute to the description of Czech as a foreign language.