Number Forms: 한, 두, 세, 네, 스무

Korean counts things, people, ages and hours with the native Korean numbers (하나, 둘, 셋…). When one of these numbers stands before a counter such as 개, 명, 살 or 시, five of them get shorter:

• 하나 → 한 (한 개, one thing)
• 둘 → 두 (두 명, two people)
• 셋 → 세 (세 시, three o'clock)
• 넷 → 네 (네 마리, four animals)
• 스물 → 스무 (스무 살, twenty years old)

The same happens at the end of longer numbers: 열하나 → 열한, 스물둘 → 스물두, 서른셋 → 서른세. 스무 is only for exactly 20, so 21 is 스물한, never 스무한. All other numbers keep their form: 다섯 개, 여섯 명, 열 살. The full forms (하나, 둘, 셋) are used when you count aloud or when the number stands alone.