english is my second language, but I learned it early enough that I didn't have to study the rules (and it more or less replaced my mother tongue).

Years later a friend who was learning english asked me the difference between "a" and "an" and I realised I had no idea. I know when to use each, but not the rule governing why.

Fascinating how different the experience learning a language can be when do you explicitly, through learning the structure and rules, versus via immersion where you know what's right, but perhaps not why.