the II chord followed by the IV chord in a major key?

Question

I am analyzing the music of [Name of Life arranged by Makiko HIroHashi] (https://musescore.com/user/4622501/scores/5038709).

From measure 29 - 30 it goes from :

(First half Measure 29 ): Eb minor [ii] (second half Measure 29): Eb major [II] (Measure 30): Gb major [IV]

I was wondering why does the II -> IV work? Is it some type of augmented6th chord, secondary dominant, or enharmonic progression that I am not seeing?

Answer 1

In the score you provide I see Eb9 in measure 29 (highlighted in red), notes Eb-G-Bb-Db-F (there is no Gb, so no Ebm), and Gb/Ab in measure 30, which can be interpreted as Absus. Bbm follows in measure 31.

Eb9 is a dominant chord; it is a secondary dominant to Absus, which is a dominant chord in the key of Db. It then resolves deceptively to Bbm, vi chord in the key of Db.