G Ionian contains the same notes as G Major: G A B C D E F#
G Dorian contains the same notes as F Major: G A Bb C D E F
G Phrygian contains the same notes as Eb Major: G Ab Bb C D Eb F
G Lydian contains the same notes as D Major: G A B C# D E F#
G Mixolydian contains the same notes as C Major: G A B C D E F
G Aolian contains the same notes as Bb Major: G A Bb C D Eb F
G Locrian contains the same notes as Ab Major: G Ab Bb C Db Eb F
From this point can we choose the appropriate substitute key (or mode), to play over a given chord or piece of music. Comparing each mode to the Ionian mode will reveal the notes that have been changed:
Dorian gives us the lowered 3rd and 7th.
Phrygian gives us a lowered 2nd, 3rd, 6th and 7th.
Lydian gives us a raised 4th.
Mixolydian gives us a lowered 7th.
Aolian gives us a lowered 3rd, 6th and 7th.
Locrian gives us a lowered 2nd, 3rd, 5th, 6th and 7th.
Using this information requires some understanding of chord structure. For instance, a minor7 chord and a dominant 7 chord have lowered 7ths. A minor 7 chord has a lowered 3rd, as well. Therefore, we can determine from above that Dorian will be a good substitution over a minor 7 chord and Mixolydian for a Dominant 7 chord.
Perhaps this will start a dialogue on uses for modes.
Here's a good link I found: http://www.guitar9.com/columnist344.html
Cheers,
SarinaJ :)