When do I use pure korean numbers and when do I use 일, 이, 삼?
The native Korean numbers are mainly used for these.
- counting objects and people in daily life. (연필 두개, 다섯 명, 집 열 채)
- hour of time (but not minute and second) (두시 삼십분, 얼두시 오분)
- counting months (일곱 달, 열 달)
The Sino-Korean numbers may be used for most things not mentioned above.
These are the things for which Sino-Korean numbers are used exclusively.
- amount of money (이십 원)
- minute and second of time (두시 삼십분 십오초)
- phone numbers (칠공구일 이륙일삼)
- measurements and mathematics (삼십 미터)
- name of the year, month, and day (이천십오년 구월 일일)
I'm sure I am going to screw this up sooner or later, but thanks for the reply.