Proper grammar involves learning and discipline; both have been abandoned in the last fifty years because they require hard work.
As a former teacher of English composition, I speak from experience: too few want to learn grammar, and too few are competent to teach it.
And this is a straightforward rule to learn. Use the subject pronoun when it refers to the doer and the object pronoun when it refers to the one on whom the action is done. For example, I gifted him a book. Apart from this, use subject pronoun for comparisons. He is taller than I. Use the object pronoun after a preposition, that is, "between you and me".
I guess we're just more of those grammar mavens the author scorns.