I would say this song doesn't have a middle eight, but that isn't really important, because you really just need a way to look at form.
The important thing to look for regarding traditional form is tonal center and phrase endings.
Let's look at a basic 32 bar song form...
32 Bar Form
A : I ... V, 8 bars
A' : I ... I, 8 bars
B : ? ... V, 8 bars (the bridge)
A' : I ... I, 8 bars
The song Over the Rainbow is an example. The
A parts use the same basic melody and only the endings change. Ending on
V for the first repeat propels the music into the second. When the melodic material is reused like this the technical term is a parallel period. The periodic structure allows for repeating the basic melodic material without it becoming played out.
B middle eight also uses the period device - it ends on
V - but it starts with some harmonic contrast. I indicate that with
? to show it could start from any number of chord. It will often be in a new key. It also is typical for
B to get new, contrasting melodic material. With three repeats of the
A melody the
B section needs to provide something new in the 32 bar form.
So, in 32 bar form periodic structure (the alternate endings on
I) propels the music forward and the
B section provides harmonic and melodic contrast.
Now for the structure of Yesterday...
A : I ... bVII7 IV I, 7 bars (1 + 2 + 2 + 2)
B : IV ... ii V7 I, 8 bars (4 + 4)
A A B
Setting aside the famous 7 bar length of the
A section, the structure is basically 8 + 8 bars with
A starting and ending on the tonic and
B moving from some tonally contrasting area back to the tonic. This is a very common form where the two sections often just alternate like
||: A | B :||. Often there is melodic contrast between
B. Unlike 32 bar form the endings are both on the tonic. But there is some similarity between the forms in that
B starts with harmonic and melodic contrast.
You could sort of view this simple form as a kind of modified truncation of 32 bar form...
I (... V, I) ... I, ? (... V, I) ... I
...where the parenthesis show us dispensing with the periodic endings and strong melodic contrast in
B. It's just 8 bars repeated where the second part starts on a new harmony and with some change in the melody.
Many songs in this simple 8 + 8 form will have a sort of 4 + 4 bar split of the
A section with a pause on
V which makes it feel like a miniature of periodic structure. I looked through my songbooks for an example that didn't do this and found the spiritual Steal Away. It's 8 + 8 bars, both
B end on the tonic,
B starts with the contrasting harmony and melody, no internal pauses on
In my opinion Yesterday doesn't use a middle 8. It's written in a different form. But it still exhibits the typical elements that delineate sections.
One final observation. The two endings in Yesterday offer a nice contrast even though both end on the tonic. The first one is a kind of "modal" ending using the
bVII chord and a plagal cadence. Harmonically that contrasts with the
B ending using a traditional
ii V I. That's nice way to set the two sections apart but still follow the basic conventions of the form.