Mastering complex sentences is essential for effective English writing, as they allow ‎students to express detailed ideas with clarity and accuracy. However, many second-‎language learners struggle with constructing grammatically correct complex sentences, ‎leading to errors that affect the overall quality of their writing. Understanding the types ‎and sources of these errors is crucial for improving students' writing skills.‎ This research analyzed students' errors in using complex sentences in English ‎composition based on the Surface Strategy Taxonomy. The study was conducted on ‎Class 5B students at Gontor for Girls Campus Three. The objectives of this research ‎were to identify the types of errors made by students and to determine the sources of ‎these errors, focusing on interlingual and intralingual transfer.‎ This study employed a qualitative descriptive method, with data collected from students' ‎English compositions. The errors found were classified into four categories based on the ‎Surface Strategy Taxonomy: omission, addition, misordering, and misinformation. The ‎findings revealed that misinformation and omission errors were the most frequent, ‎indicating students' difficulties in constructing complex sentences correctly. Furthermore, ‎the sources of these errors were identified as interlingual transfer, where students applied ‎structures from their native language to English, and intralingual transfer, which resulted ‎from incomplete knowledge and overgeneralization of English grammar rules.‎ The study concluded that students need more practice and explicit instruction on ‎complex sentence structures to minimize these errors. Future researchers are encouraged ‎to explore more effective teaching strategies to enhance students' grammatical accuracy ‎in English writing.‎