Here's something I just learned....and it might sound confusing....but...
There are 2 ways to think of modes.
1. Modes within a particular key
2. Modes for a particular note
With the first one, if you're in C, then all the modes for C will consist of notes within the C major scale....
But, with the second, all the modes will be based on notes within the key a major third below that note, so if you want to play all the modes for the NOTE C, you'd build major scales off each note in the Ab major scale, but starting on C. Just confused you?
Watch the notes.
C Ionian - C-D-E-F-G-A-B-C (C major scale)
C Dorian - C-D-Eb-F-G-A-Bb-C (Bb major scale)
C Phrygian - C-Db-Eb-F-G-Ab-Bb-C (Ab major scale)
C Lydian - C-D-E-F#-G-A-B-C (G major scale)
C Mixolydian - C-D-E-F-G-A-Bb-C (F major scale)
C Aeolian - C-D-Eb-F-G-Ab-Bb-C (Eb major scale)
C Locrian - C-Db-Eb-F-Gb-Ab-Bb-C (Db major scale)
If you look at the bold notes, you'll see that they follow the Ab major scale. This should make it really easy to play the modes for a particular note, if you don't know them all by heart, in all keys.
Extra questions....
So, now that I know this, why is this true? What does it mean?...I don't know that part yet, but I'm sure that once I answer those two, the answer will lead me to something else that I won't fully understand.