The mandatory subjects for the UPSC Civil Services Mains exam include Essay, General Studies Paper I (History and Geography), General Studies Paper II (Governance, Constitution, Polity, Social Justice), General Studies Paper III (Technology, Economic Development, Agriculture, Security), and General Studies Paper IV (Ethics, Integrity, and Aptitude). Candidates also have to choose a literature subject as one of their optional papers. Overall, a thorough understanding of these subjects is essential for success in the Mains examination.