Dude, I was still confused too. 
O.K., maybe this can help unconfuse you, if u study it. Anytime you use a mode other than Ionian/major mode (dorian, phrigian, locrian), it implies this:
1. You are not in that key, you are playing the notes of a major scale other than the starting note of that mode. Example:
C Dorian; you are not in the key of C, not using the notes of the C major scale
B phrigian; you are not in the key of B, not using the notes of the B major scale
So, how do u find what key you are in? You use the name of the mode to help you. Here is the major scale formula for reference purposes:
1st WS 2nd WS 3rd HS 4th WS 5th WS 6th WS 7th HS 8th/1st
Dorian = starts with the 2nd scale degree of the major scale; look at the major scale formula above and see where the 2nd scale degree is, to find the key, which is the 1st, take the starting note and count back 1 whole step or count up 5 whole steps. Examples:
C Dorian, take C and count back 1 whole step or count up 5 whole steps, Key of Bb
F# Dorian, take C and count back 1 whole step or count up 5 whole steps, Key of E
Ab Dorian, take C and count back 1 whole step or count up 5 whole steps, Key of Gb
Phrigian = starts with the 3rd scale degree of the major scale; look at the major scale formula above and see where the 3rd scale degree is, to find the key, which is the 1st, take the starting note and count back 2 whole steps or count up 4 whole steps. Examples:
C Phrigian, take C and count back 2 whole steps or count up 4 whole steps, Key of Ab
F# Dorian, take F# and count back 2 whole steps or count up 4 whole steps, Key of D
Ab Dorian, take Ab and count back 2 whole steps or count up 4 whole steps, Key of Fb/E
Lydian = starts with the 4th scale degree of the major scale; look at the major scale formula above and see where the 4th scale degree is, to find the key, which is the 1st, take the starting note and count back 2 whole steps + 1/2 step or count up 3 whole steps + 1/2 step. Examples:
C Lydian, take C and count back 2 whole steps + 1/2 step or count up 3 whole step + 1/2 step, Key of G
F# Lydian, take F# and count back 2 whole steps + 1/2 step or count up 3 whole steps + 1/2 step, Key of C#
Ab Lydian, take Ab and count back 2 whole steps + 1/2 step or count up 3 whole steps + 1/2 step, Key of Eb
Mixolydian = starts with the 5th scale degree of the major scale; look at the major scale formula above and see where the 5th scale degree is, to find the key, which is the 1st, take the starting note and count back 3 whole steps + 1/2 step or count up 2 whole steps + 1/2 step. Examples:
C Mixolydian, take C and count back 3 whole steps + 1/2 step or count up 2 whole steps + 1/2 step, Key of F
F# Mixolydian, take F# and count back 3 whole steps + 1/2 step or count up 2 whole steps + 1/2 step, Key of B
Ab Mixolydian, take Ab and count back 3 whole steps + 1/2 step or count up 2 whole steps + 1/2 step, Key of Db
Aeolian/minor = starts with the 6th scale degree of the major scale; look at the major scale formula above and see where the 6th scale degree is, to find the key, which is the 1st, take the starting note and count back 4 whole steps + 1/2 step or count up 1 whole step + 1/2 step. Examples:
C Aeolian, take C and count back 4 whole steps + 1/2 step or count up 1 whole step + 1/2 step, Key of Eb
F# Aeolian, take F# and count back 4 whole steps + 1/2 step or count up 1 whole step + 1/2 step, Key of A
Ab Aeolian, take Ab and count back 4 whole steps + 1/2 step or count up 1 whole step + 1/2 step, Key of Cb
Lorcian = starts with the 7th scale degree of the major scale; look at the major scale formula above and see where the 7th scale degree is, to find the key, which is the 1st, take the starting note and count back 5 whole steps + 1/2 step or count up 1/2 step. Examples:
C Locrian, take C and count back 5 whole steps + 1/2 step or count up 1/2 step, Key of Db
F# Locrian, take F# and count back 5 whole steps + 1/2 step or count up 1/2 step, Key of G
Ab Locrian, take Ab and count back 5 whole steps + 1/2 step or count up 1/2 step, Key of Bbb/A
Any questions???
