Thanks so much that really clears things up. For the part where you reference 'Yesterday', in simple terms, you say that the melody remains more or less constant (or the emotional essence of the melody), but the chords behind it go into a relative minor progression, so the even though the melody is the same, how it relates to the chords, and the feel of the song in general, changes? Am I making sense or do I have no idea what you're talking about?

Also we can tell when a song is having a brief moment of relative minor when the chord that is being worked towards, and arrived at (for a brief moment in the song at least), is the relative minor chord? Thanks again this is really helping me out