Im learning basic music theory and chord progression then I stumbled upon this song (You are my song by Martin Nievera) :
From my understanding, the first 2 lines are simply C major chord progressions with pattern: I vi IV V .
But the 3rd and 4th lines are confusing. How did Eb sounded proper in the sequence from the C major pattern? My guess is it because its part of C minor scale? What makes it more confusing is at the end, the C major scale connects again smoothly. Eb is not even part of C major's median and dominant scale degrees (Em and G). What could be possibly happening in this one and what part of music theory this concept belongs? Thanks!