Ironically, I am unclear on whether their use of "cannot" and "unable" means that some people could not, even in principle, understand these rules of grammar, o

Ironically, I am unclear on whether their use of "cannot" and "unable" means that some people could not, even in principle, understand these rules of grammar, or just that there are people who currently do not understand them, but could be taught. (Meta-ironically I am now wondering if the perceived lack of clarity is really revealing something about myself. :) )