The UPSC Prelims examination is basically conducted for the purpose of screening only. Thus if you clear the Prelims exam, you will be eligible for the Mains exam.

Also, your Prelims marks will not be considered in your final order of merit, provided you manage to make it through to the interview round.

The UPSC prelims examination comprises 2 papers, namely the General studies paper and the CSAT paper or the Civil Services Aptitude Test.

UPSC Prelims Syllabus for Paper 1: General Studies (GS)
This GS paper or paper 1 is designed to test your overall knowledge about your surroundings and the world, and also to test how observant you are as a person, by asking questions related to current affairs.

The UPSC prelims syllabus for the GS paper consists of topics from a wide range of subjects making it extremely difficult to pinpoint the kind of questions that might be asked in the upcoming paper.

UPSC Prelims Syllabus for Paper 2: Civil Services Aptitude Test (CSAT)
The CSAT or Paper 2 is conducted with the intention to test your reasoning, comprehension, and analytical abilities.

This is because, as a Civil Servant it is of paramount importance that you are able to perceive the world and the problems therein with utmost precision and rationality.

The CSAT paper includes questions from the subjects of Reasoning & Analysis and Reading Comprehension. It also occasionally tests your decision-making skills by asking you questions pertaining to the same.

Click here to read about UPSC CSE.