This is a very difficult piece to analyze in that there are many phrases or sections that could be interpreted many different ways. Also, F# isn't the easiest key in which to analyze harmonies.
It is in simple ternary form. The first A section, "Lento," is 20 measures long and is comprised of an extended double parallel period. The first half is made up of two asymetrical phrases (4 and 3 measures long, respectively), both ending with half cadences (I'll call them 'a' and 'b' from now on). In measure 8, Chopin uses a "snakey" line in the left hand to descend down the F# sacle from C# to the tonic, which begins the second half.
The second half of this period begins with the same 4 bar phrase from the first half, varied in that the rhythm of the melody is embellished. However, the second phrase is varied in an important way - it centers first arond the IV chord (Chopin uses a V7/IV instead of I). This phrase ends with a very unsatisfying half cadence, which leads into a third phrase - thus making the period extended.
The third and final phrase has this basic progression - V/ii ii V I. The phrase is extended from the third beat of m. 18 through measure 20. This extension has a terminative feel, so the first A section is strongly closed.
The B section begins with two phrases that have the progression ii V I first centering around the dominant (E), then the subdomimant (B). This sequence feels pretty stable and these four measures are therefore expository. However, the second phrase is extended with a transitional section that lasts from m. 25 to m. 28. It ends with a PAC in F# that has an F# pedal bass.
The return of A at the Tempo I sounds like the b phrase of the first A section, because it begins with the V7/IV chord. It begins with a contrasting period this time, with two asymmetrical phrases (3 and 2 measures each). Then Chopin added an extension that has a terminative feel from measures 34 to the end.
This piece has many of Chopin's trademarks - tempo rubato, which gives the piece an improvisatory feel, and a very sensitive, lyrical melodic line. It makes me feel like I'm in a cozy living room watching a fire calmly flickering in the hearth.
Man - boringly in-depth but I was especially interested after analyzing Chopin's Mazurka. The two are very similar formatic...ally...