The example excerpt below is from the 4.2 Workshop video. I’ve retained the main melodic line and harmonies from that video; however, I’ve added a four-measure introduction, and I’ve extended the theme with additional measures.

The 4-measure introduction features two contrasting chords (i.e., stability and tension) that alternate each measure. The stable chord (F#, G, B) is a subset of a G major 7 chord (tonic), and in fact, the D natural is present in beat 3 in the melody. Still, the incomplete voicing (in the lower staff) contributes to harmonic ambiguity and tension. The melodic line in the upper staff is not a fragment of something to come. Like the Alexandre Desplat example (The Shape of Water), the introduction features a somewhat improvisatory structure and melodic contour.

Notice the slightly syncopated left hand texture (in comparison to the sustaining whole notes from the 4.2 workshop video), as well as the subtle voice leading and chord voicing adjustments to the makeup of the lower staff chords throughout the four measures. Even though the lower staff is accompanimental, it can still feature 'musical' and even 'lyrical' voice-leading. 

The main theme is heard in measure five, starting with a two-measure basic idea that features the same alternating stable and tension chords, now each lasting two beats. The basic idea is repeated with minimal variation in measure seven, and the Presentation phrase (measures 5-8) prolongs the stable/tension harmony.

The Continuation phrase features several repeated fragments from the basic idea. The harmony moves away from the tonic (and the alternating stable and tension chords), from the minor iii (B minor) and major IV (C major7), to the minor vi (E minor), to the half diminished ii07 in measure 11. This phrase is specifically designed to move away and disrupt the tonic chord until the progression reaches a cadence in measure 12. The cadence features an increase in the harmonic pacing, and several secondary dominant chords (the V65/vi and the V65/V) in order to return to the tonic of G.

The cadential motion brings us back to the tonic chord in measure 13. I've extended the ending of the theme in what might be considered transitional material leading to a potential second theme in measure 20. As the main theme resolves in measure 13, contrary melodic material emerges in the lower staff with the familiar alternating stable / tension chords.

Measure 17 features some deceptive motion to an E minor chord (note: deceptive motion often features the relative minor of tonic), followed by a stepwise descending bass line to C in measure 18, at which point we hear one final 'tension' chord that resolves to the stable tonic chord. Like the introduction, there's an improvisatory nature to the melodic writing that creates a feeling of 'wandering'. Aimlessly wandering can be effective in small doses, but if prolonged for an extended duration, the music may feel formless and not coherent. Having the tight-knit sentence structure before this moment allows for some loose-knit formal structures before and after the main theme.