When I was young, my father instilled in me to know my native language because he was worried I'd forget.
So, during the weekends, he had me do pages of workbooks he had purchased from Korean bookstores and I also had to hand-write/copy the work into empty notebooks as well to hone my writing skill.
My speaking, reading, and understanding skills are casual because if I hear or read difficult vocabularies, I won't understand what the other person has said or what I've read. Even when watching K-Dramas I had to read the English subtitles because the characters used more advanced words than I did.
But, when I spoke to Korean adults, they were usually surprised because a lot of my generation of classmates barely learned or did not learn the language in their homes. And I was always honest to the adults about my speaking and reading skills by saying I did not understand difficult words so they knew not to use them. Yet, they continued to be surprised.
By the way, my writing skill is garbage because when pronouncing or saying Korean words, the letters used to write is quite confusing for me so they're usually misspelled because there are some amount of same-sounding letters in the Korean alphabet.