The main differences between the preliminary and mains examination in UPSC lie in their structure and purpose. The preliminary exam is objective, consisting of multiple-choice questions aimed at filtering candidates through basic understanding and current affairs knowledge. In contrast, the mains exam is subjective, requiring in-depth essays and critical analyses, which assess candidates' analytical and writing abilities over a longer duration. The mains also include optional subjects, allowing candidates to showcase their specialized knowledge. Thus, preparation strategies differ significantly, necessitating focused approaches for each phase.