In this study, the effect of teaching activities prepared according to Functional Grammar on secondary school students' learning of the functions and spelling of morphemes and conjunction was investigated. To observe this effect, teaching was carried out following the principles of Functional Grammar to be used in teaching the functions and spelling of -de morpheme and conjunction. The study group research consists of 76 secondary school students and 4 Turkish teachers. This research was designed and conducted in a mixed method design, in which qualitative and quantitative research techniques were used together. The quantitative dimension of the study was in a single group pre-and post-test weak experimental model; the qualitative dimension was carried out with a case study. In the collection of quantitative data, the listening activity, which was prepared according to the fill-in-the-blank technique, and included the functions of de morpheme and conjunction, was used as a pre-and post-test. A structured interview form was used to collect qualitative data. As a result of the research, it has been observed that the general success in the functions and spelling of the morpheme and conjunction in the 6th, 7th, and 8th grades has increased significantly. It has been observed that there is a significant and positive increase in all grade levels in learning the conjunction function and separate spelling of de, which has become a chronic problem. In addition, it was determined that secondary school students were successful in learning the locative of the morpheme in the noun task, the time functions in the adverb task, and its spelling. However, secondary school students could not make progress in the case function and spelling of the-de morpheme in the adverb task. Teachers and students, whose opinions were taken in support of these positive results, stated that the teaching activities prepared according to Functional Grammar are fun, instructive, permanent, and enable active participation in the lesson. Accordingly, the teaching of the spelling of the morpheme -de in the conjunction functions of locative, tense, and its conjunction started to be given from the 5th grade; it has been suggested that the case function of the morpheme and the teaching of spelling skills should be postponed to secondary education. Thus, a functional teaching method should be adopted gradually in teaching the functions and spelling of-de morpheme and conjunction.
