Mandatory subjects for the UPSC Civil Services Preliminary Exam include General Studies Paper-I and General Studies Paper-II (also known as the CSAT, or Civil Services Aptitude Test). General Studies Paper-I covers areas like history, geography, polity, economy, environment, and current affairs, while CSAT tests quantitative aptitude, reasoning, and comprehension skills. Candidates must secure the minimum qualifying mark in CSAT to be eligible for the Main Examination, making performance in these subjects crucial for overall success.