What is a Mode ?
I intend to keep this simple by not using any long Greek words here.
You could substitute the word 'mode' with the word 'mood' - different musical modes may express different moods.
Hopefully , having got this far into the tutorial , you are now well up on what constitutes the key structure of C Major.
The seven notes and their respective seven chords can each become a root note or root chord to create seven modes of C Major.
Nothing else changes - the scale and all the chords stay the same.
A common songwriting device is to pretend to be in one Key when really you are in another - this is one way of using modes.
So , for example , you could have a song that starts with an F Major chord then goes to E minor then to C major then back to F Major.
This may sound like it's in the key of F Major but really it is a mode of C Major.
The scale being used would be a C Major scale but starting and finishing on the note F
Another example would be a song or piece of music having just one chord - G7 - throughout - this is obviously in the key of G but a G Major scale won't sound right .
This is because the chord G7 belongs to the Key Structure of C Major and the piece of music is actually using a Mode of C Major.
The scale you should use in this case would be a C Major scale starting and finishing on the note G
 Play a D minor chord then play a C Major scale starting and finishing on the note D
[D - E - F - G - A - B - C - D ]
Move on to an E minor chord and do the same but starting and finishing on E.
Keep doing this with all seven chords and you should get a feel of the seven modes of C Major.
 Write an eight bar chord sequence starting and finishing with D minor using any other chords from the C Major structure then put a melody on top using the C Major scale from D to D.
 If you feel that you really understand what a mode is and want to know what the Greek word for each of these seven modes is you can easily find that information on the web.