While learning both Japanese and Korean, I have noticed an interesting tendency to have two different number systems: a “native” number system and a Chinese-imported one. In some contexts the native system is used, and in some contexts the Chinese-imported one is used. For example for Japanese, the numbers are like so from 1 to 3:
|Ichi いち||Hito ひと|
|Ni に||Futa ふた|
|San さん||Mitsu みつ|
While in Korean:
|Il 일||Hana 하나|
|I 이||Dul 둘|
|Sam 삼||Set 셋|
In Japanese, the Sino-Japanese words tend to get used for most things, but you might use the native Japanese numbers for things like counting one or two people (hitori, futari respectively) or counting “generic” things (hitotsu, futatsu, mittsu, etc) up to ten. In Korean, Sino-Korean numbers are used for phone numbers or telling minutes of time, but native Korean numbers are used for things like age, number of people or telling the hour of time. The native Korean numbers are also often attached to counting words.
In other words, there’s no real logic or clear-cut rule about when to use Chinese-imported numbers and when to use native numbers. Sometimes one system is used. Sometimes the other system is used. The Chinese-imported numbers are usually more commonly seen because they tend to be very logical and easy to pick up, while the native counting systems in Japanese and Korean are somewhat harder and less intuitive.1 For example in Japanese, the native Japanese words are often used up to 3 or maybe even 2 and never beyond 10. Meanwhile, in Korean, most young people tell their age with Sino-Korean numbers rather than the native numbers because it’s hard to remember how to say numbers like 40 (maheun, 마흔) and 50 (swin, 쉰). Older Koreans will still use the native numbers for age, but you hear that less and less.
This may sound somewhat strange to have two number systems, but English is vaguely similar. A lot of words in English are imported from Latin and Greek (mostly through medieval French). So, we say “light”, but when used as a compound word, we say “phot-” as in “photography”, “photosynthesis”, etc. English has only one number system, but in other contexts, we do have a combination of “native” words and “imported” words for the same thing. Knowing when to use either one is something you just learn.
Languages in general are messy and dynamic. There’s a lot of history behind words and rules that the average person may not be aware of, and it’s not really necessary for day to day life. But for nerds like me, it is fun.
1 I think I remember that Vietnamese is the same way, but I can’t quite remember (I studied it 10 years ago). If I recall right, the usual numbers in Vietnamese are of Chinese origin, while the native system is obscure and mostly used in old literature, but I could be wrong.