The main differences between the Preliminary Exam and Mains in UPSC lie in their structure and purpose. The Preliminary Exam is an objective type test that serves as a screening phase, designed to assess basic knowledge and comprehension of the subjects. In contrast, the Mains consist of subjective answers, evaluating the candidate's ability to articulate thoughts clearly and demonstrate in-depth understanding. The Mains also cover a broader range of topics, requiring critical analysis and essay writing. While the Preliminary Exam focuses on general studies and aptitude, the Mains emphasizes personal insights and application of knowledge.