I understand how the modes sound different by targeting certain notes. That part is coming together aurally. What I don't understand is how a chord progression can be model. Since chord progressions seem to pull from any diatonic chords, if I play a C Dm and Em , I can be playing the I ii and iii in Ionian or the VI,I and ii in Dorian. And they dont seem to always target the root except in the last Cadence some times. Who's to know ? Or are there progressions that are normally played in one mode or another. I hope I made sense...
ii-V-I is a 'progression' in the sense that there are tensions and resolutions. Basically dominants wanting to reach tonics. That's the basis of all Common Practice harmony, and it relies on using modes that HAVE a dominant chord containing the leading note of the key. That's the B in the middle of G (or G7) that wants to progress to C.
And that's why the great bulk of Western art music used major and minor keys. Those are the modes that HAVE a fully-functional dominant chord. You can go out on harmonic journeys and return on a string of secondary dominants. You can keep setting up new tonal centres by introducing THEIR dominant chords. You can turn a dominant 7th chord inside out with ♭5 substitutions - but it's all still basically subdominants, dominants and tonics.
In a mode with no proper dominant chord, this sort of functional 'progression' doesn't work. We have to work more on the basis of 'this chord sounds pretty next to this one!' When there's no place a chord definitely WANTS to go, we cant surprise by going somewhere else! That's fine, but we lose the functional drive that might be what you're thinking of as 'progression'.
You might be confused by the fact that the common meaning of "cadence" is to have a tension resulting from a dominant-functional chord, which has to resolve in another chord that (possibly) attenuates that tension, while possibly using notes from the diatonic scale of the tonal center (or its alterations).
Well, that's true for tonal music.
We are normally used to the concept of cadence as "return to the tonic in some way", but that's just its common meaning in harmonic lingo.
Tensions exist everywhere, not only in their classical meaning: a cadence is not really an end, but just a motion from A to B. And A and B could even just be two different sounds, and even unpitched sounds.
In fact, if you think about it, you are talking about "progression", and its etimology comes from the Latin word "progressio", which means "go forward".
Take for instance the classic "So what" example.
There is no actual dominant chord, you only have two chords, they both have minor sevenths, but they also are minor chords (so, technically not really dominant-ish), and they are both of the same type.
Some would say that there actually is a "normal" tension given from the Eb chord, which might resemble the harmonic function of the Neapolitan chord (which is built on the lowered second), or any augmented-sixth based chord built before the dominant - it is a half-tone-based harmony, and nothing creates more tension than a half tone.
But in reality, time - as always - is the most important factor in music.
The importance of a cadence is also created by its rhythm, the relation between the duration (and, eventually, harmonic function) of its chords and their timing (order).
Consider that having a prolonged tonic chord also creates tension, even if we don't feel it: as soon as the harmony changes, there is motion, and we realize that there actually was some level of tension (no, not because we were bored ;-) ).
Take the following:
It takes 6 minutes of tonic to get to an actual new chord. Maybe we didn't feel that need at first (and the crescendo and distorsion surely helped), but a change is always interesting after that much time.
The very concept of "So what" could be applied with any chord in the structure A-A-B-A, possibly creating a very similar effect, and that's not due to the nature of those chords, but the time relation between them.
In these cases, the cadence (or, better, tension) is given from the duration of the first chord, which creates a base, the introduction of the second, which creates an alteration (a.k.a. tension/motion), and returning to the original base (the tonic), no matter what chord was before, finalizes the progression, as its resolution stabilizes the previous movements.
I ii and iii in Ionian or the VI,I and ii in Dorian...Who's to know ?
Let contrast tonal versus modal harmony.
Tonal harmony is in large part organized around root movement by descending fifths. In jazz it's progressions like
ii V I. Another term for such progression is cadential harmony. In terms of diatonic chords and major/minor keys, the identity of those chords is not ambiguous.
Dm G C would be
C: ii V I while something like
Em: VII III VI would be "weak" from a tonal perspective where the emphasis is on primary chords
IV I V.
Modal harmony is not focused on primary chords
IV I V. I suppose you could take the approach that any progression of chords that doesn't emphasize the primary chords
IV I V or descending cadential progressions like
ii V I could be modal. In such a case something other than cadential harmony needs to define the tonic.
C Dm Em doesn't have cadential progression to define a clear tonic. It could be
C ionian: I ii ii,
D dorian: VII i ii,
E phrygian: VI VII i, etc. etc. Indeed, who's to know?
One way to clarify the tonic is with rhythm and phrasing. When could simply add bar lines at appropriate places to demark a tonic. Putting tonics at beat one immediately after the bar line is an effective way to do it...
C ionian: | I | ii ii,
D dorian: VII | i | ii,
E phrygian: VI VII | i
...or if we rotate the chord ordering and for more emphasis use harmonic rhythm to play the tonic chords for double duration it could be...
C ionian: ||: ii ii | I I :||,
D dorian: ||: ii VII | i i :||,
E phrygian: || : VI VII | i i :||
Melodically you can effectively define the tonic with lines build upon steps to the tonic scale degree, ex descending
^3 ^2 ^1 or neighbor motion
^1 ^7 ^1. Simple formulas like that work in both tonal and modal style. I don't mean the melodies would literally be only three note ideas. That tones would just be the main notes, or guide notes, of melodies.