Originally Posted by: Carl King

Schlegel is saying the song is in the key of A Major.

D minor in that context is a iv (minor iv chord). It would usually be IV (Major IV chord).

It's just a temporary non-diatonic chord for effect (or a temporary modulation).[/quote][p]Exactly. Thanks for clarifying!

[quote=Carl King]One chord (one note) is a small enough difference that it doesn't totally disrupt the tonality of the overall piece. It's just a momentary chord that strikes our ear for effect.

Right. In fact, a modulation, a note or chord that is outside of the key of the rest of the song, is often the most interesting or part of the song. It can provide variation or tension that can be released by returning to a note or chord that is in the key signature.