I'm currently learning basic music theory and along the way I review old songs on my playlist then analyze what key, chord progression and somehow discover the composer's way of creating music.
The specific song "214 by Rivermaya" is clearly written on G major with chord progression of I, vi, ii... in its intro.
What confuses me is the use of E and A major chords in the verse. I just recently explored the concept of "chromatic mediant". Based on its concept, E is the parallel key of G major's sub-median note (E minor). But the A major chord doesn't look like a chromatic mediant since it belongs to supertonic of both G major and G minor scale. What could be the explanation that describes the structure of the verse? Should I also look in the concept of modes or other music theory concepts to easily understand how songs are composed? Thanks!