Earlier quoted context omitted.
I've never understood how someone could master a subject and yet be unable to answer any questions about it. How about spoken English? I know a great many people who are able to express themselves clearly and gramatically - surely mastery of spoken English - but ask them what the rule is for order of adjectives or some such and many of them would even need to double check what an adjective is.
That's hardly being "unable to answer ANY questions"; try asking them which is more common: "the big red car" vs "the red big car", do you think they'd be unable to answer?
The question of this nature about the English language would be "how do I know what order to put adjectives in?" and the answer would be something along the lines of "opinion-size-age-shape-colour-origin-material-purpose".
I do think they would be unable to answer. The rule of course then goes deeper, into ablaut reduplication and so on. People master spoken English without knowing anything [0] about it.
[0] Yes, I know, I said "anything" and now your literalist side is jumping in joy at being able to say "Aha, aha, they DO know at least one thing and therefore your statement is not literally correct!" I don't care.