[quote name='timmo' timestamp='1456854602' post='2992926']
If I play an A minor, and use all of the natural notes, I am also playing the notes for a C major. When does it become a mode, as I think I understand that a mode is a scale just starting on a different note, but keeping within the notes of the scale. So if I start a G Major, but start it from F#. Am I playing a mode?
[/quote]
No I dont think so. Starting from G would make it a Mixolydian.
https://en.wikipedia.org/wiki/Mode_%28music%29#Ionian_.28I.29