Originally Posted by: Gilbyesk

The lesson was fund 2, chapter 2, section 3 and A major.

The backing track has both a D and Dm chord. Took me a bit to figure out.

Yes, that D minor is a minor iv chord in the key of A major. Because the D minor chord isn't in the key of A major (specifically the F note, making it a minor chord) that makes it a modulation, a brief change of key signature. It's a farily common modulation used to make a melancholy, or bitterweet sound.

The Beatles used it quite a bit: In My Life, Nowhere Man, If I Fell, Blackbird, and others.

One of my favorite uses in a pop song is Sleepwalk!

https://www.guitartricks.com/tutorial.php?input=2450