Korean has two sets of numbers, its own native numbers and numbers borrowed from Chinese. Both sets are in use up to 99. Above that, usually only the Sino-Korean set is in use.
Numbers can’t normally modify nouns as in English. Counters have to be used, as in forty head of cattle.
Korean counters take Korean numerals; Chinese origin counters take Sino-Korean numerals. There are some exceptions, which we shall note.
To some extent the native numerals may be used in general situations without a counter, for example when counting out loud, and they are also frequently used to mean “years old”.
In both sets eleven would be “ten + one” and twenty would be ‘two + ten”.
Note how dates are handled, using the Sino-Korean counter, 년 'year': 천 구백 팔십사 년 1984
여덟 is pronounced /여덜/.
Some of the native Korean numerals have abbreviated forms when they occur before counters: 하나 -> 한, 둘 -> 두, 셋 -> 세, 넷 -> 네, 스물 -> 스무
열 makes a following consonant tense, if possible for example: 열둘 /열뚤/
열여섯 etc. are pronounced /열려섣/.
백만 is pronounced /뱅만/.
제 친구 중 한 사람이 채식주의자예요. One of my friends is a vegetarian.
Note that in this sentence, 사람 is a counter, not a normal noun. As 사람 is a native Korean word, native Korean numerals are used before it.
만 원이예요. It’s W10,000.
Again 원 is a counter. As it is a Sino-Korean word, it has Sino-Korean numerals before it.
술 한잔 one/a glass of rice wine. 잔 takes the Sino-Korean numerals.
Names of days and months
Occasionally, you will find exceptions to the rule, for example 권, a Sino-Korean counter is used to count books, but is preceded by the native Korean numerals. For example 책 두 권 ‘two books’.