If any non-British wonder why we Brits don’t know the rules, well back when I was at school, studying GCSE English in the 90s (qualifications we get age 16), they explicitly didn’t teach us grammar rules, at all, just made us write stories. It was a bit of an experiment that I think they have since reversed.
This meant that I knew rules of French grammar more explicitly than English. Which is nuts.
This is true for native speakers of most languages, in my experience. I remember many years ago traveling Spanish-speaking countries and trying to talk to locals about some grammatical point, like use of the subjunctive mood, and getting very puzzled looks. This despite the fact that even three-year old children correctly navigate verb moods without knowing the rules.
This and other experiences convinced me that learning the formal rules of a language is perhaps not the best way to approach learning. Especially not if your intention is to speak to locals, rather than say, become a TV news announcer. I think the most critical skill of language learning is to get the "muscle memory" of the language, the reflex of subconsciously mapping concepts to words in your internal lookup table. This skill is even precedes IMO listening comprehension: naturally, when you're listening to someone speak, you'll miss words. But if your mastery of the language is sufficient to allow you to quickly guess what you would have said in the gap if you were speaking, then the error is survivable. I find that this seems to be how the brain works when listening to speech in your native language.
To correlate with Dan Ariely's "Thinking, Fast and Slow" it's the fast mode that's needed, whereas as a grammatical approach tends to emphasize and develop slow mode skills. Sometimes you need to go slow before you go fast, but given that children jump I without knowing the rules, I don't think it's necessary for language. (I also think too much is made about the supposed inelasticity of the adult brain re languages) I hope/expect with AI, there is a really interesting possibility around the corner for learning language in a simulated immersion environment. The Google and Amazon translation APIs seem, on paper, to provide at least 50% of what that would take.
You were lucky. In my day, we were still being taught that the grammar of Latin somehow applied to English. There is a species of English usage that can make a certain sense as English, but which more or less follows rules that can be applied when learning Latin or Greek. That set of rules was taught in schools for a couple of centuries because the aim was to teach the student Latin at least, and perhaps Greek. Somewhere along the way, people who had been taught that system began to believe that this Latinised English represented proper English grammar. It's only recently that the educational consensus has become, "well, that's a load of old bollocks." The first comprehensive descriptive grammar of the actual English language wasn't published until 2002 (The Cambridge Grammar of the English Language).
This meant that I knew rules of French grammar more explicitly than English. Which is nuts.