I was analyzing "Try to Remember" from the musical "The Fantasticks" and became stumped on what seems to be a flatted seventh major chord stuck in the middle of the song. I'd like to understand what's going on and how it fits in with the rest of the nearby chords.

In this arrangement, it's the F major chord at the end of the fourth line. Are we in a separate key at this point? Is this just a substitution for another chord?