Although everyone has what Johanna describes, that "ability to understand language automatically" seems to be parcelled out somewhat unequally.  Several decades ago, I sat in on an English Methods class at Harvard, and I'll never forget the professor telling the students (all of whom were brilliant) that when they went on to teach in the junior and senior high schools of America they would always find a student or two who knew more about the grammar of English than they did.  I was quite surprised at the time, but after twenty years of observing English teachers teaching, I know he was right.
I've seen the same phenemenon when I teach teachers: there is always someone who "knows" more about grammar than I do.

Ed Schuster 
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