all i know about scales and modes is that the scales sound different, not only because they are on different notes, but because they have different patterns.
for instance...let's say you wanted the phrygian mode, this mode has a different pattern than the mixolydian.
they're called intervals. and the intervals for the phrygian mode are
HWWWHWW (W is whole step, meaning two frets) (H is for half step, meaning one fret)
ok. so the intervals for mixolydian are
WWHWWHW
the patterns are much different.
I hope I helped and didn't confuse you any more than you already were.