The aim of this study is to reveal the effect of foreign language learning on mother tongue use. Case study design, one of the qualitative research designs, was used in the study. In order to keep the sample large, 70 pre-service teachers were selected using simple random sampling method among the 4th year undergraduate students studying in three different foreign language education departments in 2016-2017. The research data were analysed using deductive analysis method. As a result of the research, the pre-service teachers who participated in the research think that language learning is a necessity and that individuals learn languages out of necessity. The pre-service teachers state that interest in the target language is an important condition for language learning, that if there is no interest, the language cannot be learnt, and that belonging to a common language family increases interest in the language and facilitates language learning. The desire to communicate with other individuals in the world, to improve oneself, to learn the culture of the target language and to live abroad are among the reasons for learning a foreign language. Learning the education system and sound harmony of the target language and economic concerns are among the reasons for language learning. The pre-service teachers who participated in the study stated that they had the most difficulty in learning basic language skills in foreign language learning and that they made comparisons with their mother tongue while learning a foreign language. The pre-service teachers who participated in the research stated that they pay attention to the use of the mother tongue while learning a foreign language; they pay attention to the use of the mother tongue while learning a foreign language because they benefit from the love of the mother tongue and translations. [This research was presented as a summary paper at the RESSCONGRESS International Educational Sciences Social Sciences Symposium held in Bandirma.]