Native Korean and Sino-Korean numbers are used in different circumstances, which can make learning them a bit frustrating! As a really basic rule (that doesn't always apply), Native Korean numbers are used for counting things and talking about age, and Sino-Korean numbers are used for things like money, dates, and measurements.
There are 2 counting systems in Korean: pure-Korean numbers (하나, 둘, 셋, etc) and sino-Korean numbers (일, 이, 삼, etc). Sino-Korean numbers come from Chinese numbers (一 (yī), 二 (èr), 三(sān), etc). The number system you know is the sino-Korean numbers and here we are using pure-Korean numbers (하나,etc)