We were taught English grammar pretty well, I think, but I think I learned an awful lot more from the foreign languages I did at school - Latin, French, German, and for the final 3 years, Russian.
I later taught English as a Foreign Language, mainly to speakers of Arabic, and this was a whole new thing again. As a native speaker you don't have to learn the rules for your own language - you absorb the vast majority of them naturally. (I don't mean the things pedants like me get worked up about, like 'should of' instead of 'should have', but just the way the everyday language works.). Our 'bible' as teachers of EFL was a grammar for the purpose - a very different thing from a grammar for native speakers.
When I once had to take someone else's higher level class, I had to teach them 'the third conditional'. Although I had completed a very well regarded TEFL course I still hadn't the faintest idea what that was until I. looked it up in my 'bible', when I realised that it was something we use every day, but is quite complicated tense-wise for the poor old learners.