We learn language usage at a young age just by hearing it spoken - essentially rote learning. We may or may not understand the structure of the language. We were forced to do French somewhere in high school. In the very first class the teacher started with the French versions of the verb to be - "Je suis, il est, elle est, tu es, etc". I was completely lost because I didn't understand the concept of the verb to be in English - I don't think we had never been taught it! I just used it as I had heard it.
At some point in my adult life I had to learn the difference between affect and effect because I had never properly understood that one was a verb and the other a noun. I also had its and it's wrong - for some reason it seemed natural to me that the apostrophe version was the possessive and maybe a teacher never got through to me - I can remember being taught it. To this day I still subconsciously write it the wrong way and have to remember to check.
I used to go to Indonesia surfing for many years and started to learn some Indonesian language. I learnt more about the structure of the English language in that process than all the English classes at junior high school - after which I was happily allowed to drop English and stick to maths / science.