I don't have any problem remembering the natural Korean numbers up to 12, as they are used in telling the time - and it's easy to work out those up to 19. But I have problems remembering 스물 (20), 서른 (30), 마흔 (40) and so on. Is there any underlying meaning or other logical way I could remember these numbers more easily when I use them so rarely?
흔 means ten in obsolete Korean. These words still have the influence of '흔'.
- 일흔 여든(여흔->여든)
And 열(10), 스물(20) don't follow this rule because the origin of these two words are totally different from others. There are other 'pure Korean' numbers like 온(100), 즈믄(1000) but modern Koreans do not use them anymore. Only from 열 to 아흔 are used now. So it is much easier to just memorize them because you have to remember only nine cases. With the examples I showed you, we can easily figure out that pronunciations and notations have changed a lot from its original form. So if you try to remember it with any logic, it may confuse you more.