verb ないと vs なければならない 뭐가 격식 차이가 있나요

Question

친구가 문자로 「もう行かないと」라고 보냈는데 문법책에는 항상 「行かなければならない」로만 나와있어서요. 두 개가 같은 뜻인가요?

Accepted Answer

Same core meaning ('must'), but ないと (short for ないといけない) is very casual/conversational — friends texting, spoken Japanese. なければならない is the full, more formal/written form you'll see in textbooks, announcements, rules, etc. Your friend just dropped the ending because in speech, everyone understands 「もう行かないと(いけない)」means 'I gotta go now'. Both are grammatically 'must', just different registers.