Funny...
I was reading on modes today and really found it hard to understand (and i gathered that a lot of other people do too). So I thought to myself 'fuck it, just play something'. So I played the first note my finger fell on and started messing around. I didn't even know what scale I was playing in and definitely not caring what notes I'm hitting as long as they sound good together. But I accidentally hit a nice melody and wrote the notes down.
And I'm looking at it and thinking: Root note: F. F sounds good. And it sounds nice too, definitely major not minor. But there's a discrepancy here... Instead of just a Bb I also have an Eb. This shouldn't be here... but how come it sounds good still. So I put a repeating F bass line on GuitarPro and lo and behold, it sounds good even with the extra Eb in there.
Then I think some more. OK, so maybe it's not in F. Maybe it's in Bb Major, or G Minor. Those have Bb as well as Eb in their scales. But as I put the basslines on either of these notes, the melody just doesn't sound good enough. Because, too me, F is definitely the root note of this melody not Bb or G. So what now?
Suddenly it strikes me. I Google 'guitar modes' quickly and hope to see an answer: And there it is! Mixolydian Mode. It contains a flattened 7... that elusive Eb!
So I, accidentally, stumbled upon the Mixolydian Mode! And now that that's happened I can say I understand (roughly) what modes are! They're just a simple twists on basic scales. Like spices and herbs.
Congratulations on arriving at a true understanding of the modes and for arriving at it through your own creative and musical efforts. The theory (via Google) just told you the name of the mode in question (Mixolydian) but you actually felt that F was the key note among the note set you were using, which led you away from mistaking it for Bb major or G minor, both of which have the same notes as F Mixolydian but don't have F as the key note.
just a small question...
does the bass line have to be in F Mixolydian now?
It doesn't have to be, but if you want to retain the appealing Mixolydian flavour created by your melody, then you don't want to be contradicting those notes with a bass line that owes its existence to another scale.
What creates a modal sound is not just the notes you're playing, but also the notes that are NOT played. For example, F Mixolydian has the note Eb and doesn't have the note E. If your melody contained both E and Eb, it wouldn't have the Mixolydian sound, although it might be great music.
So, if you want to emphasise the song's "Mixolydian-ness", don't weaken it by adding non-Mixolydian notes to the bass or chords, (except perhaps as brief passing notes, etc.)
