F7 is very obviously the I chord in “Hey Jude,” because it sits in the metrically strongest position in the phrase, and it ends the phrase too. Due to the influence of the blues, dominant seventh chords feel perfectly stable. In rock, though, the rules are different. Also, in classical, you can’t use a dominant seventh chord as a tonic because it feels unstable. In common-practice-era classical music, and jazz too, the clearest way to determine what key you’re in is to look for V-I cadences. Some classical theorists see the “Hey Jude” progression and say, well, you’ve got an F7 and a B-flat, so clearly we’re in the key of B-flat, and the F7 is the V7 chord. You want an endless flow, rather than a narrative with a beginning and end. In loop-based music like rock, this is a good thing. The IV-I progression doesn’t have that sense of finality that V-I has it’s more of a comma than a period. In this direction, each step represents a IV-I plagal cadence. Rock does that too, but it’s just as likely to move its roots clockwise on the circle. In classical music, and jazz too, the roots of chords tend to go counterclockwise around the circle of fifths. The repeated melody in bars two and three works like the repeated last two lines of “Stopping by Woods on a Snowy Evening” by Robert Frost. It’s similar to the literary device of using the same words in different contexts with different meanings. This idea of the same note sequence having different meanings in different harmonic contexts is a powerful compositional strategy. The rhythmic syncopation makes these multiple meanings possible. It falls on the notes E-flat, D, and C, which are the fourth, third and second of the B♭ chord, and the flat seventh, sixth and fifth of the F7 chord.
Fourth bar: The “He-ey Jude” phrase starts in on the tail end of the third bar, and the word “Jude” anticipates the downbeat of the fourth bar by a sixteenth note.G and F are the sixth and fifth of the B♭ chord, and the stable-sounding fifth is the one that gets held out. The melody in the third bar is exactly the same as the melody in the second bar, but the chord is different. Third bar: Here’s where the real beauty part is.It turns the E-flat chord into E♭(add2), a richly jazzy sound. And yet, the F is is the note that gets held out for most of the bar. F is more surprising it’s the second of the E-flat chord, not part of the basic triad. The chord contains G as its third, so it’s a perfectly logical melody note.