Something I've been trying to get an answer to for a long time:
How do you know where to APPLY these scales?
One way to apply the modes to your improvisations involves how the chord
functions in the piece. The function may be natural (evolving from the chords placement in the scale of the piece) or temporary (evolving from the chords placement in a temporary key).
The first function to consider is the Dominant 7th (G7 in C or C7 in F) etc. When using modes of the major scale, they will call for the mixolydian mode regardless of the overall key. A dominant 7th chord that is not a natural part of the key of the piece is in essence temporarily taking us to another key. Even in the key of C major, if you run across a D7, you will use the D mixolydian (G major scale played from D to D). The primary feature of the Mixolydian mode is that it contains the major third of the chord and the lowered seventh. More exotic modes that work with a Dominant 7th in different situations will contain those two notes.
The half diminishied chord (m7b5) naturally falls on the seventh degree of the Major scale or the second degree of the natural minor. A half-diminished chord takes the locrian mode - the major scale starting on its 7th degree (B - B in C major). It almost always acts as either a temporary VII or a temporary II (derived from minor).
The minor seventh usually takes the dorian mode (major scale starting on second degree: D - D in C major) In most instances it is acting as a II or temporary II as in the II - V - I (which is functionally similar to VI - II - V; III - VI - II; VII - III - VI). This is the case in most instances in contemporary popular styles and jazz.
So even an Em7 in C major, if followed by some kind of A chord is actually acting as II of D, not III of C.
Occasionally the III or the VI is actually acting in its natural function within the key (such as a progression of I - II -III - IIIb - II - V - I) in this case the III would take the phrygian mode. Similar, the VI may require the Aeolian mode. This is where you may need to experiment a little to get the sound that you are looking for. But in general, you are usually safe in first trying dorian mode.
One way to think about it is that dorian is your primary mode for minor chords. Aeolian is Dorian with a flat sixth degree, phrygian is dorian with a flat six and a flat two.
Finally we get the Major Chord. These fall naturally on the First degree and the fourth degree. This gets a little tricky but I find that more often than not, a Major Chord has a tendancy to act as I or temporary I and therefore uses Ionian (the Major Scale) In a progression of I - I7 - IV, the I takes the Ionian and in most instances the IV will take the Ionian as a temporary I (made so by the I7). In C major C - C7 - F : The C is Ionian in C, the C7 is mixolydian, and the F is ionian in F).
When the IV chord is acting in its natural setting as IV then it will take Lydian -- which is Major with a sharp fourth degree. This will take some experimentation. A major seventh with #11 will always call for lydian (Cmaj7 #11) regardless of function.
When using the ionian mode, the 4th degree is considered an
avoid tone which means that it should not be used in a prominant position (on a strong beat or a held note) because it represents the sus 4 and is in conflict with the 3rd.
I hope that this gives you some ideas in working with modes.
