I never actually studied English grammar. The bulk of my early English learning took place in CompuServe's chat rooms (much like IRC), two decades ago, and in various virtual worlds. I vividly recall my utter confusion when people used "ain't", and I just couldn't figure out its precise usage, because it was used in so many different, inconsistent ways.
It took a while until it dawned on me that it was slang!
In a way, that was an almost natural way of learning, just in cyberspace instead of meatspace. As a result, I use English grammar "intuitively", which isn't always entirely correct, but oh well! That's what editors are for!