When writing a melody in the Mixolydian mode, you don't want it to sound as if it were in a major key. So how do you go about writing melodies in the Mixolydian mode, while maintaining the essence of the modality?
^7 scale degree is lowered (flattened.) Some features or qualities that will provide...
- Generally it is one scale step in the direction of the minor mode, so that's a kind of "darkening" of mood or color.
- Considering the primary triads - tonic, subdominant, dominant - the dominant become minor. A minor dominant is significant as it clearly moves out of the standard major/minor system.
- When the
^7is lowered the degree name changes from leading tone to subtonic, the triad built on the subtonic is a major triad which contrasts with the diminished leading tone triad in the major scale. That means the
^7triad becomes a stable chord.
- If the normally raised
^7leading tone is characteristic of the major/minor system - with DO-TI-DO being a common melodic/cadence pattern, then the lowered
^7in that melodic pattern using the lowered
^7(solfege TA) helps make the modal quality clear: DO-TA-DO.
I don't know what style you intend to write in, but you might want to look for some folk tune examples that are in Mixolydian mode. That is fairly common and shouldn't be too hard to find either at online folk music sites or in folk song anthologies. Sometimes those resources are even indexed by key/mode which will make finding examples easy.
You flatten all the 7ths but avoid settling on the subdominant chord too much.
But really, why set out to write in Mixolydian? Write a tune, maybe it will turn out to be Mixolydian. Think 'Oh listen, that's Mixolydian. Interesting...' then carry on.
Maybe it will stay in Mixolydian, maybe not. Quite likely it will 'mix' a lot of modes. A basic blues starts out feeling a bit Mixolydian. Then we get a flattened 3rd. Then a dominant, complete with leading note. We know where the tonic is. Mode is pretty well up for grabs.