Counting 1–99: Assembling Tens and Units

Once you know the native ones-digits (하나·둘·셋…) and the native tens (스물·서른·마흔…), the entire range from 1 to 99 is already yours — you just have to click the pieces together. The good news is that the assembly is completely regular, with none of the memorize-every-word pain of the tens themselves. The one thing that trips up almost every learner is not the building; it's what happens the instant a counter follows. This page shows you both: how to assemble any number, and the single load-bearing rule for putting it in front of a counter.

The assembly rule: ten first, unit second, written solid

To make any number from 21 to 99, you write the native ten and then the native unit, joined into a single written word with no space between them.

The tens word never changes when you add a unit to it — 마흔 stays 마흔 whether it's standing alone (40) or carrying a unit (마흔다섯, 45). Think of the ten as a fixed prefix and the unit as a suffix that slots in behind it.

This differs from English in a small but real way: English forty-five keeps a hyphen and two audible words; Korean fuses the two morphemes into one unbroken spelling unit, 마흔다섯, and pronounces them as one word. There is no "and," no connector — just ten then unit, jammed together.

Now the rule that actually matters: only the last unit changes before a counter

Here is where learners lose points, so read this section twice. When a counter follows the number, the shape-shift that native numbers famously undergo (하나→한, 둘→두, 셋→세, 넷→네) applies only to the final unit. The tens word in front is untouched.

Notice what does not happen: 스물 does not shrink to 스무 here. The 스물 → 스무 reduction only fires when 스물 is the whole number sitting right before the counter (스무 살, 20 years old). Bury 스물 inside a compound like 21 and it keeps its ㄹ: 스물한 개, never ×스무한 개. The change lands on the unit 하나 → 한, and 스물 rides along unchanged. (The standalone 스물 → 스무 story lives on the forms that change before counters.)

When the final unit is 5, 6, 7, 8, 9, or 0: nothing changes at all

The shape-shift rule has a quiet flip side that makes life easier: only the units 하나, 둘, 셋, 넷 change (plus the standalone 스물). Every other unit — 다섯 through 아홉 — is frozen. So if your number ends in 5, 6, 7, 8, or 9, the whole thing stays exactly as you built it, even with a counter behind it.

This is genuinely reassuring: the numbers you're most tempted to over-correct — 47, 99 — need no correction at all. Leave them alone.

Bare counts vs counted phrases, side by side

A useful habit: learn each number in its two guises — the bare count (no counter behind it) and the counted phrase (with a counter). The bare count keeps the dictionary shape of the unit; the counted phrase applies the shift.

That last sentence puts the same number, 23, in front of two different counters (분 and 개) — and both times the shift is identical: only the unit 셋 → 세, with 스물 untouched. Once you see that the counter type is irrelevant to which part changes, the rule stops feeling fragile.

Common Mistakes

1. Changing the tens word instead of the unit. The tens word never inflects; the unit does.

• ✗ 스무한 개 (shrinking 스물 inside a compound) / 서르세 명 (mangling 서른)
• ✓ 스물한 개, 서른세 명 — seumulhan gae, seoreunse myeong — "21 things, 33 people"

2. Failing to change the final unit before a counter. This is the top error the brief warns about — leaving the dictionary shape in place.

• ✗ 스물 셋 개, 서른 넷 명
• ✓ 스물세 개, 서른네 명 — seumulse gae, seoreunne myeong — "23 things, 34 people"

3. "Correcting" a unit that doesn't change. 다섯 through 아홉 are frozen; don't invent short forms.

• ✗ 마흔일 개 (butchering 일곱), 아흔아 개 (butchering 아홉)
• ✓ 마흔일곱 개, 아흔아홉 개 — maheunilgop gae, aheunahop gae — "47 things, 99 things"

4. Splitting the number with a space. A bare compound count is written solid, as one word.

• ✗ 스물 하나, 마흔 다섯
• ✓ 스물하나, 마흔다섯 — seumulhana, maheundaseot — "21, 45"

5. Reaching for Sino numbers with a native counter. Objects, people, and age (casual) take native numbers all the way to 99.

• ✗ 사십칠 개 (Sino 47 + 개)
• ✓ 마흔일곱 개 — maheunilgop gae — "47 things"

Key Takeaways

• Build any number 21–99 as native ten + native unit, written solid: 스물하나, 마흔다섯, 아흔아홉. The assembly is fully regular.
• The tens word never changes — not when adding a unit, not before a counter. 마흔 stays 마흔.
• Before a counter, only the final unit shape-shifts, and only if it's 하나·둘·셋·넷: 서른세 명, 스물한 개.
• 스물 → 스무 only as a whole number (스무 살); inside a compound it stays 스물 (스물한 개, not ×스무한 개).
• If the number ends in 5–9 or 0, nothing changes before a counter — 마흔일곱 개, 아흔아홉 권, 예순 장 are all left as built.
• Native numbers cover 1–99 only; at 100 you switch to Sino (the 100+ switch).