Native Tens: 스물, 서른, 마흔, 쉰…

If the native ones-digits (하나·둘·셋…) felt manageable, brace yourself: the native tens are where this system asks the most of your memory. In the Sino set, twenty and thirty are transparent little sums — 이십 is literally "two-ten," 삼십 is "three-ten," and once you know 십 you can build every ten for free. The native set gives you no such gift. Each ten is a completely separate, unpredictable word — 스물, 서른, 마흔 — with no visible thread back to its ones-digit. 마흔 (40) shares nothing with 넷 (4). You cannot derive them, only learn them. The payoff for learning them is large, though: these are the words every Korean uses to say their age, so they come up in the first five minutes of countless conversations.

The nine native tens

Here is the full ladder. Treat every row as its own vocabulary item:

Look at the two right-hand columns together and you see the deepest divergence between Korea's two number systems. The Sino column is a machine: 십, 이십, 삼십, 사십 — one root, plus a multiplier. The native column is a list: nine unrelated words you simply hold in memory. This is the moment where "just learn the Korean numbers" fully collapses as advice — there is no rule to extract, only lexemes to store.

Filling the gaps: ten + unit

The tens combine with a ones-digit to make everything in between, and here the logic is regular: the ten comes first, the unit second, written solid.

So once the tens are memorized, the in-between numbers cost nothing extra. The only thing to remember is that the unit still obeys the shape-shift rule when a counter follows — 서른셋 becomes 서른세 명, because the final 셋 → 세 (see the forms that change before counters).

Where the native tens earn their keep: age

The native tens' star role is age. When a Korean tells you how old they are in ordinary conversation, they use a native number + the counter 살 — and past twenty, that means a native ten.

Age also drives the honorific counter 세 with Sino numbers in formal settings (see age: 살, 세, 연세), but the everyday, spoken default is native — and that keeps these nine words in constant circulation.

The tens also count people and other tangible groups, though for larger crowds many speakers slide over to Sino:

One reduction to remember: 스물 → 스무

Among all nine tens, exactly one changes shape before a counter: 스물 becomes 스무 (스무 살, 스무 명, 스무 번). Every other ten keeps its full form — 서른 살, 마흔 살, 쉰 살, all unchanged. This is the same reduction 스물 undergoes everywhere, and it is a frequent slip precisely because the neighboring tens don't do it.

Notice the contrast in one breath: 서른 keeps its shape before 살, but 스물 shrinks to 스무. If you can hear that difference, you have the reduction down.

The ceiling: native runs out at 아흔아홉

The native ladder climbs to 아흔아홉 (99) and then simply stops — there is no native word for one hundred. So a lifetime counted in native numbers hits a wall exactly at the century mark and hands off to Sino 백:

Look at that single sentence: 아흔아홉 (native, 99) in the first half, 백 (Sino, 100) in the second. The switch is forced by the system, not by style — there is no ×아흔열 or any native hundred to reach for. The full account of this handoff is on where native runs out.

Common Mistakes

1. Building the tens on the Sino pattern. There is no ×두열 ("two-ten") or ×삼열. Each native ten is its own word.

• ✗ 두열 (for 20), 삼열 (for 30)
• ✓ 스물 (20), 서른 (30) — seumul, seoreun

2. Trying to derive a ten from its ones-digit. 마흔 is not built from 넷; memorize it.

• ✗ (reasoning) 넷 → so 40 must be something like ×넷흔
• ✓ 마흔 — maheun — "forty" (an unrelated, memorized word)

3. Forgetting that 스물 alone shrinks to 스무. Twenty years old is 스무 살.

• ✗ 스물 살이에요.
• ✓ 스무 살이에요. — seumu sarieyo — "I'm twenty." (but 서른 살, 마흔 살 keep their shape)

4. Using a Sino number with the native counter 살. Casual age wants a native number.

• ✗ 삼십 살이에요.
• ✓ 서른 살이에요. — seoreun sarieyo — "I'm thirty." (formal register instead uses Sino 삼십 세)

5. Pushing native numbers past 99. The native ceiling is 아흔아홉; a hundred is Sino 백.

• ✗ 아흔아홉 다음은 아흔열
• ✓ 아흔아홉 다음은 백 — aheunahop da-eumeun baek — "after ninety-nine comes a hundred."

Key Takeaways

• The native tens — 열·스물·서른·마흔·쉰·예순·일흔·여든·아흔 — are nine separate memorized words, not a "two-ten / three-ten" construction like Sino 이십·삼십.
• Don't derive them: 마흔 has no link to 넷; treat each ten as pure vocabulary.
• They combine regularly with a unit (스물셋, 서른다섯, 마흔아홉); the final unit still shape-shifts before a counter (서른세 명).
• Their main job is casual age (서른 살, 예순 살) — high-frequency, everyday speech.
• Only 스물 → 스무 reduces before a counter; the other tens keep their shape. And the native ladder stops at 아흔아홉 (99) — a hundred is Sino 백.