In short

Korean runs two number systems at once. Native numbers (하나, 둘, 셋 …) count hours, ages and things through a counter word; Sino-Korean numbers (일, 이, 삼 …) do minutes, money, dates and anything large. The whole skill is picking the right set for each counter - and remembering that on the clock the hour is native (세 시, se si) but the minute is Sino (삼십 분, samsip bun).

The rule

Step 1 - Two number systems, and Sino counts by ten-thousands

Sino-Korean numbers are borrowed from Chinese: 일 이 삼 사 오 육 칠 팔 구 십. They stack cleanly - 23 is 이십삼 (isipsam, literally two-ten-three), 45 is 사십오 (sasibo). The higher units are 백 (baek, 100), 천 (cheon, 1,000), 만 (man, 10,000) and 억 (eok, 100,000,000). Korean groups large numbers by 만 (ten-thousands), not by thousands, so 100,000 is 십만 (simman, ten 만), 1,000,000 is 백만 (baengman), and you never build a separate “hundred-thousand” word. Sino numbers do minutes, money, phone numbers, dates, floors, bus routes (칠 번 버스, chil beon beoseu, the number 7 bus) and any large count.

Step 2 - Native numbers, and the five truncations before a counter

Native numbers run 하나 둘 셋 넷 다섯 여섯 일곱 여덟 아홉 열, then the tens 스물 (20), 서른 (30), 마흔 (40), 쉰 (50), 예순 (60). They fade out around 100, where Korean switches back to Sino 백. Five of them lose their tail the moment a counter follows: 하나->한, 둘->두, 셋->세, 넷->네, 스물->스무. So it is 한 개, 두 명, 세 시, 네 살, 스무 살 - never *하나 개. Only these five change; 다섯, 여섯, 일곱 and the rest stay as they are (다섯 개, 여섯 명).

Step 3 - Counters, and the pattern that carries them

You cannot count a Korean noun bare - you count it through a counter (단위명사), in the order:

Noun + Number + Counter -> 사과 세 개 (sagwa se gae, three apples), 커피 두 잔 (keopi du jan, two coffees).

The noun comes first, then the number, then the counter, all with spaces. Most counters are locked to one number system: 개, 명, 마리, 장, 권, 잔, 병, 살, 시간 and clock-hour 시 all take native numbers; clock-minute 분, plus 년, 월, 일 and 원 (won), take Sino numbers. 번 is the one that does both - native when it counts times (세 번), Sino when it labels a route or an item (칠 번). The full assignment is in Forms & patterns.

Step 4 - The clock: native hour, Sino minute

This split is the classic beginner trap. The hour is a native number + 시 (si), the minute is a Sino number + 분 (bun):

native + 시 + Sino + 분 -> 세 시 삼십 분 (se si samsip bun, 3:30).

반 (ban, half) replaces the minutes: 세 시 반 (se si ban, half past three). Add 오전 (ojeon, a.m.) or 오후 (ohu, p.m.) in front: 오후 다섯 시 (ohu daseot si, 5 p.m.). A time point takes the particle 에: 두 시에 (du sie, at two) - but note that a duration does not (Step 6).

Step 5 - Dates: all Sino, with two spelling quirks

Year, month and day are year 년 + month 월 + day 일, every part a Sino number: 이천이십육 년 칠월 십팔 일 (icheonisimnyuk nyeon chirwol sippal il, 2026, July, the 18th). Two months change shape to smooth the sound: 6월 is 유월 (yuwol, not *육월) and 10월 is 시월 (siwol, not *십월); the others are regular (일월, 이월, 삼월 …). Weekdays are a day-word + 요일 (yoil): 월요일, 화요일, 수요일, 목요일, 금요일, 토요일, 일요일. To ask the date, use 몇 월 며칠 (myeot wol myeochil, what month, what day) - and note the word for “what day of the month” is spelled 며칠, never *몇 일.

Step 6 - Duration

For how long, the counters are 시간 (hours, native), 일 (days, Sino), 주일 (weeks, Sino), 개월 (months, Sino) or its native twin 달, and 년 (years, Sino): 세 시간 (se sigan, 3 hours), 십 일 (sip il, 10 days), 삼 개월 = 세 달 (sam gaewol = se dal, 3 months), 이 년 (i nyeon, 2 years). A stretch of time is wrapped with 동안 (dongan, for, during), which takes a space: 세 시간 동안 (se sigan dongan, for three hours). Do not put 에 on a duration - *세 시간에 공부해요 is wrong.

Step 7 - Range and frequency particles

Five short markers ride on the number words. 부터, 까지 and 마다 are particles and 씩 is a suffix, so those four attach with no space; 동안 is a noun, so it keeps its space:

• N + 부터 … N + 까지 - from … to: 한 시부터 세 시까지 (han sibuteo se sikkaji, from 1 to 3), 월요일부터 금요일까지.
• N + 동안 - for a duration: 이 주일 동안 (i juil dongan, for two weeks).
• N + 마다 - every: 날마다 (nalmada, every day), 주말마다 (jumalmada, every weekend).
• N + 씩 - each, apiece: 하나씩 (hanassik, one each), 조금씩 (jogeumssik, little by little).

Forms & patterns

The two systems, 1-10 and beyond:

Truncation before a counter (only these five):

Counters and the number system each takes:

Assembling a time and a date:

Examples

Common mistakes

• 오 살이에요. (O sarieyo.) -> 다섯 살이에요. (Daseot sarieyo.) - Age with 살 takes native numbers, so five years old is 다섯 살, never the Sino *오 살.
• 둘 개 주세요. (Dul gae juseyo.) -> 두 개 주세요. (Du gae juseyo.) - 둘 truncates to 두 the instant a counter follows; the same goes for 하나->한, 셋->세, 넷->네, 스물->스무.
• 삼 시 삼십 분이에요. (Sam si samsip bunieyo.) -> 세 시 삼십 분이에요. (Se si samsip bunieyo.) - On the clock the hour is native (세 시) but the minute is Sino (삼십 분). Do not run the whole time in one system.
• 육월 십오 일이에요. (Yugwol sibo irieyo.) -> 유월 십오 일이에요. (Yuwol sibo irieyo.) - June is the irregular 유월 with the ㄱ dropped; October is likewise 시월, not *십월.
• 오늘은 몇 일이에요? (Oneureun myeot irieyo?) -> 오늘은 며칠이에요? (Oneureun myeochirieyo?) - “What date” is one fixed word, spelled 며칠. The spelling *몇 일 does not exist.

Tips

• Two feels, one rule of thumb. Native for things you touch and small close counts - people, cups, animals, ages, and clock hours; Sino for measured or labelled numbers - money, dates, phone numbers, and clock minutes. When in doubt on the clock, chant “hour native, minute Sino”.
• Truncation lands on the word right before the counter, so inside a compound it falls on the tail. 20 alone is 스물 and 20 years old is 스무 살; 25 keeps the full form, 스물다섯 살 (the counter follows 다섯, which never truncates), but 21 truncates again - 스물한 살 - and so does 31, 서른한 명. The teens behave the same way, which is exactly where the clock needs it: 열한 시, 열두 시, never *열하나 시.
• 분 is two different words. As a native-number counter it is the honorific for a person - 한 분, 두 분 (one/two people, politely); as a Sino-number counter it is minutes - 십 분, 삼십 분. Context and the number system tell them apart.
• Counters glue in the order Noun + Number + Counter. It is 커피 두 잔, not *두 잔 커피 and not *두 커피. English “two coffees” reorders; Korean keeps the noun in front and hangs the count behind it.