CLIL fosters linguistic and cognitive development by creating learning environments where the target language (L2) serves as the natural medium for communication and subject-specific instruction. This linguistic environment is further enriched by a third dimension (L3), where L2 interacts with the native language (L1), subject-specific terminology, and symbolic representations (L3s). This third dimension plays a particularly significant role in the CLIL classroom and contributes to language comprehension.

The success of CLIL largely depends on the quality of classroom interaction. Subject teachers must therefore prepare meticulously, carefully managing the language they use in the classroom to support students' language development in the foreign language.

A case study from an Italian science high school illustrates these dynamics. A mathematics lesson delivered in English was analyzed, structured into three distinct phases. The introductory phase involved the English teacher distributing bilingual materials along with a specialized L3 vocabulary list. In the central phase, the mathematics teacher explained the lesson objectives using both L2 and L3, supported by visual aids. Finally, the concluding phase required students to provide written feedback on the lesson.

The analysis of classroom discourse in this CLIL setting reveals its complexity and dynamic nature, incorporating multiple text types. The teacher's discourse primarily consists of procedural, descriptive, explanatory, and argumentative texts, each exhibiting varying degrees of complexity. Textual complexity is assessed based on referential accessibility, informativeness, cohesion, coherence, syntactic subordination, iconic ordering, and cognitive load. In this CLIL setting, L2 is predominantly used during the introductory and concluding phases, whereas the central phase relies heavily on L3 and L3s. Instructive texts provide clear, step-by-step guidance, while descriptive texts offer essential contextual details. To enhance comprehension and reduce cognitive demands for students with limited proficiency in English as a foreign language (EFL), the mathematics teacher employs brief, paratactic sentences, frequent pauses, and non-verbal cues.

Overall, the findings suggest that a streamlined, multimodal communicative approach in CLIL not only enhances mathematical reasoning but also facilitates smooth transitions between natural and symbolic languages, thereby improving both language acquisition and content comprehension.

Keywords: Classroom discourse analysis, CLIL (Content and Language Integrated Learning), English-mediated Instruction, Systemic Functional Linguistics (SFL) Textual complexity.