Korean counting is very similar to english counting. 21 is twenty-one.

But there are two ways to say numbers.  Or write it out.

I don't really know how to explain this... but one number system is used sometimes, while the other number system is used other times.

일 (il), 이 (ei), 삼 (sahm), 사 (sah), 오 (oh), 육 (uek), 칠 (chil), 팔 (pal), 구 (gu), 십 (ship).  This is mosty for math. Like when you are explaining a math problem.  And money. 

하나 (han-ah), 둘 (dool), 셋 (sait), 넷 (nait), 다섯 (da-sut), 여섯 (yeo-sut), 일곱 (il-gop), 여덟 (yu-dul), 아홉 (ah-hop), 열 (yeul).  This is for counting. And age. 

I really don't know how I can explain this, but if you have any questions, remember to ask me!!! :)