The key differences between the Preliminary and Main Examination in UPSC lie in their structure and purpose. The Preliminary Exam consists of two objective-type papers that primarily test candidates' knowledge of general studies and aptitude. It serves as a screening mechanism. In contrast, the Main Examination includes nine papers, mainly of descriptive type, focusing on a deeper understanding of various subjects, including optional subjects and essay writing. Additionally, the Main Exam assesses candidates' analytical abilities and depth of knowledge, making it a more comprehensive evaluation than the Preliminary stage.