The UPSC mains exam significantly differs from the preliminary exam in both structure and purpose. The preliminary exam consists of two objective-type papers, primarily testing general studies and aptitude. In contrast, the mains exam includes nine descriptive papers that evaluate candidates' subject knowledge and analytical abilities in greater depth. The mains also focuses more on the optional subject chosen by candidates, allowing for specialization, while the preliminary exam is broader in scope, aiming to shortlist candidates for the mains. The mains is more rigorous and comprehensive, emphasizing writing skills and critical thinking.