Textbook vs Real
Textbook밤이 길었습니다bam-i gil-eot-seum-ni-da
Real밤이 길었다bam-i gil-eot-da
Why Koreans say it this way
Uses ~다 (plain/literary form) for dramatic effect. Korean dramas use this to sound poetic or introspective. You'll never hear 길었습니다 in a dramatic monologue — it sounds like a weather report.
From: 이태원 클라쓰 EP7