Grammar used to be a core part of grammar school. It was for me, along with penmanship. I'm not (that) old, my school was weird, but one is expected to graduate high school knowing what indefinite and definite articles are, which is which in English, and the distinction they represent.

Explicitly rather than implicitly, I mean. I believe this is still expected of high school graduates in America, yes? So the use of the negative particle in the first sentence isn't indicated.