The first thing you must do is look at the major scale. Each degree of the scale has a varying degree of weight. What I mean by that is that the note wants do take you somewhere, or it wants to stay where it is. From the scale we build chords, which have the same basic properties as the single notes in the scale.
Lets start with the 1 (the tonic) and the 5 (the dominant). 1 is home. It is referred to as the tonal center; this note defines the key. All the other notes have the qualities they do because of how they relate to the 1. Usually at the end of a piece of music the harmony resolves to the 1. If you are playing a song in the key of G major, usually you will end on the G major chord. When you are there it feels complete.
The 5 however is a very active tone. Try playing a blues in G major and instead of ending on G stop at D (or D7). You will find That it doesn't sound complete. It will scream to you "GO TO G!" It wants to resolve to the tonic.
All the other notes hves specific characteristics. The 2 wants to go to 1. The 3 wants to go to 4, the 4 wants to go to 5 or back to 1. The 7 wants to go to one. This forms the basis of harmony. I really recommend you take at least of year of harmony class. It may seem like drag writing in the style of and analyzing Bach, but it will make you better musician. It will increase your sensitivity to the qualities of your medium...sound.
What makes a mode different than its parent scale is that the tonal cnter is shifted to the 1st degree of the modal scale. The notes behave differently, the chord tones are different, although technically it is the same notes.
Example. In the key of C major, G7 is the 5 chord. G7 is derived from the C major scale. Have a friend play a C chord while you slowly play a c maj. scale. Listen carefully to each note and how it sounds over the chord. Now have your friend play a G7 chord. Play the C major chord again and listen carefully. Pay particular attention to the c note. You will find that i woks great over the C chord...It's the root note. But over the G7 it has a very different quality. It is the 4th (or 11th) in realation to the G or root note. It will sound quite tense, it will want to resolve to the b note (the 3rd of the chord). Same key, same scale, but quite different effect. This should give you some idea of what makes modes important.
There are alot of books out there on this, but I strongly recommend study. If you have a guitar teacher start asking questions. If not you should get one. If your teacher doesn't understand this stuff you may want to consider getting a new one. Learn the basics, the nuts and bolts of music. Then you can work towards playing anything you want.
Email me if you have any other questions I may be able to help you with.
