Watch the Open D and D7 Chords online guitar lesson by Marcy Marxer from Mandolin for Guitar Players
The open D chord is played 2, 0, 0, 2 which gives you the 3rd, 5th, root and another 5th in intervals. Play it with your middle two fingers. D7 is played as 2, 3, 0, 2 which gives you a 3rd, a b7, root and 5th. Play it by adding your pinky on the second string 3rd fret.