The difference between the UPSC Preliminary and Mains examinations lies in their structure and purpose. The Preliminary exam is an objective-type test focused on screening candidates, primarily assessing their general studies and aptitude through multiple-choice questions. In contrast, the Mains exam is subjective and emphasizes in-depth knowledge through essay-type questions across various subjects. The Mains assess candidates' understanding, analytical skills, and ability to express ideas coherently. Consequently, success in the Preliminary does not guarantee advancement to the Mains, necessitating comprehensive preparation for both stages.