iTutor Musics Updates

In a book called "Harmony: a course of study" by G.W. Chadwick, it is said about the ii chord:"The most important of these triads is founded upon the supertonic of the scale, and, being the parallel minor of the subdominant, its third is often doubled, especially after a tonic chord."Let's see if I'm thinking correctly (using the Cmaj scale): ii - D F Ais founded upon the supertonic of the scale: Yes, it is. D is the supertonic of the Cmaj scale.being the parallel minor of the subdominant: Subdominant is F => F G A Bb C D E F and its parallel minor is D E F G A Bb C D. I think this sentence is somewhat misleading, or my english is not good enough. Is it saying that the third of the supertonic triad is the tonic of the parallel minor of the subdominant (which is not correct because it should be the D and not the F) or it should be understood as "considering the parallel minor of the subdominant"?Thanks