The significant differences between the Preliminary and Mains stages of the UPSC exam include their structure and focus. The Preliminary exam consists of two objective-type papers aimed at screening candidates, while the Mains comprises nine papers, including subjective assessments that evaluate depth of knowledge and analytical abilities. Mains require extensive essay writing and answers that demonstrate critical thinking, whereas Preliminary focuses more on general awareness. Additionally, the Mains stage carries higher weight in the overall scoring, necessitating deeper preparation strategies.