Counting unit.

(1) There are various counting units in the Korean language. '개', which means 'piece, item, object', is widely used when counting things and '명', which means 'person', is used when counting persons. '분' and '사람' are also used when counting persons, but '분' is the polite form and shows respect for the persons counted. These counting units do not occur as independent words, but are connected to numbers. The Korean
numbers are used with these units, for example '다섯 개, 열 개', and '일곱 명, 아홉 명', and the name of things or persons should precede these expressions.
시계 다섯 개 five watches
책 일곱 개 seven books
학생 열 명 ten students
선생님 열 여덟 분 eighteen teachers
Several Korean numbers are changed when they used with counting units.
Learn more about Korean alphabet here: Korean Alphabet

Korean Numbers Number + counting unit
하나 (1) : 한 개, 한 명, 한 분, 한 사람
둘 (2) : 두 개, 두 명, 두 분, 두 사람
셋 (3) : 세 개, 세 명, 세 분, 세 사람
넷 (4) : 네 개, 네 명, 네 분, 네 사람
스물 (20): 스무 개, 스무 명, 스무 분, 스무 사람
사과 한 개 주세요.
Give me one apple.
저는 아이들이 세 명 있어요.
I have three children.

Other counting units will be introduced later. Both Chinese and Korean units are used according to the counting units.

(2) Both Korean and Chinese numbers are used when speaking about time is said. Here are some examples:
04:40
K.N : C.N.
네 시 사십 분
Korean number + 시 (o'clock)
한 시 one o'clock
열 시 ten o'clock
Chinese number + 분 (minutes)
사십 분 fourty minutes
삼십 분 thirty minutes
한 시 반에 만납시다.
Let's meet at 1:30.
('반' is used often as thirty minutes)
수업이 열 시 오 분에 끝났어요.
The class ended at 10:05.

Learn how to read in Korean here.