Textbook vs Real
Textbook걱정하지 마세요geok-jeong-ha-ji ma-se-yo
Real걱정하지 마geok-jeong-ha-ji ma
Why Koreans say it this way
Ri Jung-hyuk's protective line. 마 (casual "don't") vs 마세요 (polite "please don't"). Dropping the polite 세요 shows closeness and authority — he's protecting her, not being a stranger.
From: 사랑의 불시착 EP4