While improving a learner's language skills during native language and foreign language teaching, the learner should also be taught language rules. Language rules can be taught via grammar topics. One of the grammar topics is case suffixes. Not using any or none of these affixions causes a break in the semantic connection between words and a disruption in sentence structure. Case suffixes are topics to be emphasised on and require attention and elaboration while teaching foreigners Turkish because of their effect on spoken and written expressions with their semantic connection providing function. The aim of the study is to examine the mistakes done by foreigners learning Turkish in case suffixes (nominative, locative, ablative, dative and accusative). The study is qualitative. The data were collected via document review and the data resource was selected via the purposeful sampling method. The data resource for the study consists of written expression papers of a total of 69 learners. As a result of examining the papers, it was determined that there were mistakes with regard to case suffixes in the written expressions of the learners from each level, and most mistakes were done in the accusative case and fewer mistakes were done in the ablative case.