Language plays an important role in communication, particularly in multilingual environments where speakers use more than one language in their daily interactions. In the context of EFL students, English may be incorporated into Indonesian and Mandailing communication through code-mixing. This study investigated English language adaptation among EFL students at SMA Negeri 3 Panyabungan, focusing on the patterns and functions of code-mixing in daily communication. The study aimed to identify the patterns of code-mixing used by students and to analyze their communicative functions. This research employed a qualitative case study approach. The participants consisted of ten selected EFL students. Data were collected through naturally occurring conversations and semi-structured interviews and were analyzed by identifying, classifying, and interpreting the English elements used in students’ utterances. The findings showed that English code-mixing was a natural part of students’ daily communication, especially in informal interactions and academic discussions. The patterns found were insertion and alternation, while congruent lexicalization was not sufficiently evident. Code-mixing was used to express emotions and reactions, create humor, refer to academic activities, communicate spontaneously, and maintain informal interaction. Its use was influenced by school, social media exposure, peer interaction, familiarity with English expressions, and communication context in everyday school interactions.
Keywords: English Language Adaptation, EFL Students, Code-Mixing, Insertion, Alternation, Sociolinguistics.