The exposition of Mozart's String Quartet in D minor (K. 421) is quite the sonata formed mold. Dedicated to Joseph Haydn, these quartets are the cornerstone of chamber music repertoire and are frequently performed. The "Haydn" quartets are some of Mozart's most famous, including the 'Hunt' quartet (K. 458) and the 'Dissonance' quartet (K. 465).
Both themes within the exposition are packed with rhythmic intensity, and rightfully so, due to the slow tempo notated by Mozart. There is a chaconne bassline played within the first tonal area (m. 1-24), accentuated by the upper strings and their short but repeated melody. The second phrase (m. 25-40) is much different with a longer melody, more elaboration and extended types of variation, including all voices. What makes it most interesting, however, is the different types of articulation and bowing Mozart implies/directs, creating interesting texture and balance within the quartet.
The development begins in E flat, quite a stark difference from the original D minor. Two prevalent themes exist within the section, the first resembling the FTA of the A section whereas the second resembling the STA of the A section. The first again contains the chaconne-ish bass and creates disposition for the listener, pushing its way towards a welcome resolution in bar 59, a G minor chord. Here the second theme takes charge and introduces a new melody, eventually moving the tonal area back to the original key of D minor.
Finally, we arrive at the recapitulation, and as expected is back in the key of D minor and is nearly a literal repeat of the exposition. Again, as expected the B area has modulated to the key of D minor and embellishments have been altered accordingly.
Although simple and straightforward, Mozart's quartet poses considerable difficultly for the ensemble. The tempo is slow, leading to one of two problems: 1) speed up 2) slow down. Take time and use a metronome to watch those troubled spots that may lead to significant tempo fluctuation. That's not to say the entire piece should be at the same steady tempo (as liberty should be taken to emphasize cadential moments) but when that liberty and expression is introduced it is important to start in the original tempo. That being said, emphasize the difference between dotted quarter/eighth note figures and triplets. It's always been a problem and will continue to pose trouble. Plus, that difference can make or break a good performance of this piece. Alongside tempo and rhythmic accuracy, the articulation (especially unison sections) must be addressed. Careful attention to details and style will pay off eventually and can create a one-of-a-kind performance.