Where it gets complicated is that one can know how to do something without being able to explain it to oneself let alone teach it to others.
For example, as I mentioned I frequently ran into situations where I could tell whether a student's sentences were correct or not, but I struggled to explain why. One example from early in my teaching career was when students would place their adjectives out-of-order, for example "The German, red, old, large car..." instead of "The large, old, red, German car...". I intuitively knew that the former is incorrect and the latter is correct, but when students would ask me why, I struggled to articulate a reason.
But the local teachers on staff (i.e. native Chinese speakers) would chime in with "The order of adjectives in English is opinion, size, age, shape, color, origin, material, and purpose." They (the local teachers) still made mistakes in their English, but they had mostly memorized the rules from classroom study, and could recite them better than I could. Which was helpful to both us as native speakers (who wanted to give the students concrete answers to their questions) and to the students (who wanted rules to govern future scenarios they might encounter).
I was admittedly a bit cocky coming into that job, thinking I was qualified simply because I was a native speaker. I quickly learned that teaching a subject is a skill unto itself. It requires abilities like gauging levels of understanding by asking comprehension questions, and tailoring the subject matter to those comprehension levels, so as not to either talk down to the student or talk over their head.
And the implicit-order-of-adjectives thing is something I was not even aware of, but makes sense now! Fascinating!
Looking at you, the "b" in debt, that I was pronouncing for a long time growing up and learning a lot of words from reading.
Schwas everywhere randomly (why is it adjust (uhd 'juhst) and not ad 'juhst when we have accept (ak 'sept). In German this is way more consistent. Diphthongs everywhere, almost no pure monophthongs. Which is a language feature but in written form is also fucked. I tend to have problems with oh vs aa sounds. E.g. poland is pou luhnd and polish is paa lish. Stress isnt written. Consonants not only can be spelled differently but also said differently. Gif vs djif, cell vs celt, china vs machine
This makes the language way harder in a high level than it should be if it had had some spelling reform at some point. Sorry for not using IPA Im on the phone.
There's two pronunciations of 'polish' though: the one you mentioned being what one does to grandmother's candlesticks, and 'pou lish' referring to someone or something from 'pou luhnd'.
The best way to learn written English is by reading English books[1]; the best way to learn spoken English is to interact with native speakers, a lot. Build familiarity without ever trying to justify "why that was the correct [expression]". It'll just feel right when it's right, because it fits the statistical pattern your brain has picked up.
Grammar is to language like ballistics is to throwing & catching a ball.
[1]: For someone with a basic grasp of the language but not enough exposure yet, Young Adult content is often easiest to read. I read most of the Sherlock Holmes stories, some James Bond novels, and a lot of Heinlein growing up.