Ok....somewhat related to all this...I was improvising to a backing track song - Morning Dew - in D major. It obviously resolves to the D. When playing the D major scale if I hit the C# note it sounds awful. So I would flatten it to C and it sounded great. Is this perhaps because the song is in D mixolydian mode? Therefore the key of G major? And if so how would one figure that out? Just by ear or trial and error? Thanks in advamce!