Allegro ma non troppo, E major, 2/2 time

D 154, a fragment of the first movement, is written in Allegro, 4/4 time. As mentioned in the overview, although the second subject and development section share material with D 157, D 154 differs most significantly from D 157 in the ambiguity of its exposition's tonal structure. After the first subject is presented in E major at the beginning, the second subject appears at measure 30. This new subject is presented in the tonic key and then repeated, modulating to B major (m. 37), but the dominant region remains unstable, suddenly returning to E major (m. 56), or modulating through B minor (m. 59) and G major (m. 64) to C major (m. 67). The dominant key of B major is only established at the end of the exposition, where the second subject is presented again, dolce (m. 73). Thus, in D 154, the overall tonal structure of the exposition becomes difficult to discern due to the second subject being presented in the tonic key before moving to the dominant.

In the exposition of D 157, modulations to remote keys are also actively employed. While it is conventional in an exposition to place a transition section between the two subjects to prepare the dominant key for the second subject, here the introduction of the second subject is forced through sequential progression and a general pause. Even within the second subject area, the modulations from B minor to G major and C major, and the return to B major simply by shifting intervals, retain a certain awkwardness. However, compared to D 154, the tonal structure is clearer in D 157, as the first subject, with its wide range and frequent unisons, is clearly presented in E major, and the homophonic second subject is clearly presented in B major. Considering this point, D 154 can be regarded as representing an intermediate stage in the composition of D 157.