In Korean, there are two counting systems: Sino-Korean (Chinese System) and Native Korean (Korean System).
When to Use Each Counting System
Roughly speaking, we can use the two counting systems as the following table.
|0 - infinite||1 - 99|
|large numbers||small numbers|
|time: minutes, seconds|
days, months, years
|time: hour ("o'clock")|
e.g. 3 apples, 5 dogs, 7 people
Sino-Korean takes less time to pronounce the numbers, so maybe that's one of the reasons to use it for the large numbers.
Since Sino-Korean System is based on Chinese numbers, some of them have similar sounds to them. Below are a few examples of them. Click the Korean and Chinese Characters and they will pronounce them.
Basic: 0 and 1 to 10
In Sino-Korean, 0 is 영 (yeong) or 공 (gong) as "zero" or "oh" in English respectively. On the other hand, 빵 (bbang) is used to count zero in Native-Korean.
From 1 to 10, each system goes as the following table.
1 - 10
|1||일 (il)||하나 (hana)|
|2||이 (i)||둘 (dul)|
|3||삼 (sam)||셋 (set)|
|4||사 (sa)||넷 (net)|
|5||오 (o)||다섯 (daseot)|
|6||육 (yuk)||여섯 (yeoseot)|
|7||칠 (chil)||일곱 (ilgob)|
|8||팔 (pal)||여덟 (yeodeol)|
|9||구 (gu)||아홉 (ahop)|
|10||십 (sip)||열 (yeol)|
From 10 to 19
From 10 to 19, both Sino-Korean and Native-Korean pronounce the numbers with the combination of "10 + number." For example, 11 is "10 (십) + 1(이)," 15 is "10 (십) + 5 (오)," and so on.
From 20, both count the same way. But Native-Korean has a distinctive name for 20, 30, 40, ..., 90, and 100. It's explained in the following paragraph.
10, 20, 30, ..., 100
Sino-Korean counts "number + 10," but Native-Korean has different sound for each numbers.
After 100, the numbers are the combination of the above-mentioned numbers. Plus, we will have new numbers like "1000 (천) and 10000 (만)" that will also be keys to count large numbers.
(Just for clarity, I put a space between the Hangul in the below table, but we don't need the spaces in a real situation.)