Also plenty of people who are fluent in a language don't really have a good grasp of technical stuff. I used to speak my native language very formally when I was young, and didn't really make (m)any grammar mistakes, but in tests they might ask what specific form of conjugation was used in a given sentence and I wouldn't have any idea what it was called.
I also used to know English grammar, sentence structures, conjugations, conjunctions, accusatives, intransitives etc. back when I was still learning, but fuck me I haven't thought about those for decades by now, and don't remember almost any of the meanings. Doesn't mean I'm not fluent in English.