The subjects that are compulsory for the UPSC mains examination include Essay, General Studies Paper I, General Studies Paper II, General Studies Paper III, and General Studies Paper IV. In addition, candidates are required to take two optional papers from a variety of subjects they can choose based on their background and interests. The Essay paper tests candidates' ability to articulate thoughts and argue effectively, while the General Studies papers cover key areas like Indian history, geography, politics, economics, and ethics. These subjects collectively aim to assess a candidate's comprehensive understanding of national and global issues.