Mozart's String Quartet in D Minor is in Sonata Form.
The exposition takes place in measure 1-41 and starts in D minor. In measures 12-24 there is a transition that takes the second tonal area into F Major, or III, just as expected. The two tonal areas vary much more than just the key. The first is much broader while the second has more fast notes. The first seems more sorrowful and lamentish than the dancelike second. When performing this piece, I would bring these differences out by playing more rubato in the first area while playing the second as strictly what is written. There are a few parts of the exposition that contain typical quartet textures. In measures 1-6, the first violin plays a solo part while the others accompany, and in measures 32-33, the first violin and cello play a duet while the second violin and viola accompany. Additionally, in measures 12-14, a motive is passed through the quartet. In measure 106, a codetta starts to end the exposition, and follows a PAC.
Measures 42-69 are the development and retransition. The development begins in Eb major and develops the material from the very beginning of the piece. Along with the motive from measures 1-2, the motives from measures 9-10 and 25-26 are also developed in this section. Here we also see many typical string quartet textures. In measures 42-45, there is one instrument playing a solo while the other three accompany, and in measures 55-58, the first violin and cello play a duet. In measure 69 a motive is passed through the instruments, and in measure 52, the violins play a duet. Then, in measure 65, the retransition starts, which is signified through the use of frequent C sharps.
The recapitulation starts in measure 70. This time, the second tonal area stays in the tonic. Although there is a transition similar to the exposition, it doesn't modulate to F major. The second tonal area is also different as it starts two beats off than in the exposition, which may show that beats one and three were equally important when this piece was written. At the end of this section there is a repeat, and so the last bar in the first ending modulates to Eb Major to get back to the beginning of the development. The second time through however, there is a coda to end the movement.