To me, scales and modes are merely notes if played without underlying harmony. All the different modes have their sound because of the harmony they form. If we ignore the harmony, the modes will just be notes without much meaning on their own. Lets look at the mode/chord chart:
Our favorite C major scale again. Each mode corresponds to a chord and that’s what separates them from each other. So, how to use them? Lets say we are improvising over a tune in the key of C major and it has all chords diatonic to C major. What mode do we play over each chord? Let’s start it easy. Since it’s all diatonic, if we play everything using notes in C major, we are safe (presumably). Before we go around noodling over C major, here’s something more important:
Outline the chord using chord tones
For each chord we encounter, play the chord tones i.e. for Dm7, play D, F, A and C (i.e. 1 3 5 7 for each mode). Playing the chord tones for each chord is the safest way of making sure things sound right. There’s no need to play ALL the chord tones over each chord though. It depends more on the phrasing which I’m not covering here. So for each 7th chord which has 4 notes, we still have 3 notes left for each mode i.e. for Fmaj7, after outlining the chord with F, A, C and E, we still have G, B and D left from the C major/F Lydian scale. Don’t be afraid of these notes just because they aren’t chord tones. Instead:
Fill in the gaps using non-chord tones
By filling in the gaps using the non-chord tones, besides outlining the chords, we are now outlining the modes as well! Do you now see/hear how different modes sound over each chord in a song? I hope you do! If you’re playing to a track, turn it off and try outlining each chord with just single notes.
Basically, when improvising over diatonic chord changes, think of/look at the chords first, outline the chords using chord tones (you don’t have to play all chord tones!) then fill the rest with non-chord tones (again you don’t have to play all of them!). Apart from the basics, work on PHRASING. Tell a story from your solos, not ‘oh look at this Dorian mode, oh yeah you heard that?’. For starters, try improvising over Jason Mraz’s ‘I’m Yours’ which is basically a I V vi IV loop. Look out for the II7 chord that appears in some parts of the song though! As the II chord isn’t diatonic to the key, we will have to play another scale just for this particular chord. If you’re wondering what chord/scale to play over that II7 chord, play a II Mixolydian mode (Mixolydian mode with the 2 as the tonic i.e. if the chord is D7, play D Mixolydian which is also a G major scale). Now, go practice!