My first language is Korean, and one rule no one gets right is the spacing. It seems pretty obvious in English and many other languages, but in Korean, there are things that look like a single word but is actually two separate words (and vice versa), particles which needs to be written without spacing, and some other rules that makes it confusing. It’s bad enough that one of the former director of the National Institute of Korean Language confessed he’s not confident about it either.