Vocabulary expresses all the word elements that constitute a language. Learning a language is the equivalent of vocabulary level. This situation is effective in teaching a language as a second language as well as in first language acquisition. One of the determinants of the language levels during the process of teaching Turkish as a foreign language is vocabulary. In this study, it is aimed to determine the vocabularies and vocabulary coefficients in the written languages of A1, A2, B1, and B2 students who learn Turkish as a foreign language in line with the frequency of occurrence and prevalence. Structured with a case study by qualitative research, the study was designed with an embedded single-case type. The study group consists of 20 students studying in the TOMER (Turkish and Foreign Languages Research and Application Center) unit of a foundation university. Students are individuals who have been included in the levels of Turkish education from A1 level to B2 level. Student's texts in the writing section of the final exam at the end of each level constitute the study object of the research. The data were collected by document analysis and coded with units of "word, proper name, reduplication, idiom, proverb, formulaic expression." The categories were created with the analysis performed through content analysis, the qualitative data were quantificated and turned into word lists, and these data were evaluated through descriptive analysis. The study was conducted by considering credibility and transferability criteria. As a result of the analyses, a total of 3754 vocabulary elements, 742 of them being different, were identified at the A1 level; a total of 3316 vocabulary elements, 662 of them being different, at A2 level; a total of 4464 vocabulary elements, 835 of them being different, at the B1 level; and a total of 1000 vocabulary elements, 520 of them being different, were identified at the B2 level. The total number vocabulary elements obtained at all levels is 16554 and 1981 of them are different from each other. The vocabulary coefficient was determined as 0.20 for A1, 0.20 for A2, 0.19 for B1, 0.20 for B2 and 0.11 for all levels. The number of vocabulary elements with a prevalence of 4 was determined as a total of 9730, 179 of them being different; the number of elements with a prevalence of 3 was total of 2185, 180 different; and the number of elements with a prevalence of 2 was total of 1956, 361 different. The number of vocabulary elements used in only one level is 2683 in total and 1261 of them are different. The number of new words, which were used for the first time, is 375 in A2, 362 in B1, and 502 in B2. Vocabulary development is 51% in A2, 32% in B1, and 34% in B2. A limitedness was detected in students' use of reduplication, proverb, and formulaic expression compared to the use of words and idioms. The findings of the research displayed similarities with the findings of two different studies that were already conducted.
teaching Turkish as a foreign language, vocabulary, vocabulary coefficient, word prevalence