A lot has been written about modes. It's an open-ended topic that can easily get confusing and distracting from the actual goal of playing music. The first thing to say about modes, is that they are just scales. One of the modes is nothing more than the major scale, another is the natural minor scale. The others are variations created by altering various tones in the scale. Each mode has a distinct feel of its own, and a funky Greek name to go with it.And this:
The modes provide a framework within which to think about note selection. In DADGAD this framework will generally be centered around the key of D initially. Players quickly familiarize themselves with the 3rd to control the major or minor tonality of a piece. However, there is great potential for variation among the other notes and without some kind of system, note selection becomes random. A single variation such as flattening the 6th in the D minor scale makes an enormous difference to the feel of a tune.Worth studying. Also some good information, along with MIDI files, on chords. Again in DADGAD tuning.