उपसर्ग (Prefixes) are added before a root (धातु) to modify or change its meaning. Sanskrit has 22 standard उपसर्ग: प्र (forward/special), परा (away/reverse), अप (away/down), सम् (together/complete), अनु (after/following), अव (down), निस्/निर् (out/without), दुस्/दुर् (bad/difficult), वि (apart/special), आ (toward/until), नि (down/into), अधि (over/upon), अपि (also/over), अति (beyond/excessive), सु (good/well), उत् (up/out), अभि (toward/intensely), प्रति (back/toward), परि (around), उप (near/under).

Key examples: प्र+हार = प्रहार (strike), अनु+करण = अनुकरण (imitation), वि+ज्ञान = विज्ञान (science), सम्+स्कृत = संस्कृत (refined), अधि+कार = अधिकार (authority), प्रति+कूल = प्रतिकूल (unfavorable), उप+कार = उपकार (favor), अभि+मान = अभिमान (pride), परि+वार = परिवार (family), निर्+भय = निर्भय (fearless). Note: same धातु with different उपसर्ग gives completely different meanings.

REET Exam Tips: Know all 22 उपसर्ग. Most common question: identify उपसर्ग in given word, or match उपसर्ग+धातु to formed word. Key trap: 'प्र' prefix vs 'प्रति' — प्रख्यात has 'प्र' prefix, प्रतिकूल has 'प्रति' prefix.