This piece is gorgeous. Yet again does Brahms manage to make a major key nostalgic, gentle, almost softly tragic.
It is a composite ternary form where A is closed, B is open, and A' is again closed.
A is a binary form, and honestly I'm torn as to rather it's rounded or simple. For you see, a' ends in the key of A Major, but it's last four bars--texture-wise--resemble much more nearly the b section, even though part of the original a motive are interspersed and even the return of a is signaled by a variation on the original theme, a' seems too different, and b is also longer with a transition of at least three measures. For a, we have a parallel progressive repeated period with symmetric four bar phrases. The first phrase ends on a half cadence in A, the second on an IAC in E, and then repeated. B lasts for eight measures and is highly tonally unstable. For instance, immediately after the E major chord ending the a section, we have a C major (chromatic mediant) chord which is then followed by a dimished built on a c sharp which resolves to a Dominant seventh built on E (the leading tone of the diminished going to the seventh of the dominant seventh) which resolves to I'm not quite sure what. This is in the fourth measure of b, and then we begin again with another four measures beginning in F major this time (moving up and adding energy like a good development). This second four measure phrase group ends on an E major chord and moves on into three measures of transition to arrive finally at a'. We seem to have the return of the original motive; however, in the third measure it changes direction and we have two repeated subphrases containing new expository material ending the first phrase on a sort of plagal cadence in its eigth measure. We commence the next phrase with a return to the high a, which reminisces of the original a, but does not quite come through. We have this time a four measure subphrase that highly resembles the original a, but a sense of authentic cadence is deprived us as the right hand ends on a d and g on the strong beat of the fourth measure. And so we move on to a rather sequential set of measures pausing again at a weak authentic cadence another four measures on at m42 and finally cadencing strongly at the end of another eight measures in A major(m48), thus making big A closed. You could see the cadence having occured right at m38 with the first weak PAC and the rest just being an extension on that original cadence. It certainly does have a winding down feel to it after that point, and there is no more return of the original a melody after that point.
Section B of the ternary form looks actually like a little, simple ternary form. To begin, a is in f minor and ends the first four measures with a quick HC and then in a following 4m contrasting phrase modulates to what I feel to be C# major and ends on a half cadence leading smoothly back to that original c# in the beginning B melody. Thus a of B is open. Then b of B moves into the dominant minor, so you get that nice sighing effect of the C# major half cadence resolving to the parallel minor. After four bars of the b, we have an IAC cadence on c#, further weakened because it is on the second beat of the measure. Then the phrase repeats and ends now on a PAC in c#. So we have a nice parallel, symmetric period for b. And b is closed. There is then a modified return of a, beginning again on the c#, only this time an octave lower. Again after four measures we end on a HC and then after another four we end on a PAC. I call this ternary because b is stable in its tonality and complete unto itself. Also there are no transitions.
Then at m73 we have a transition of four measures into the return of A. This transition ends on a suspension of the dominant of E with the E already lying in the bass. Here a only plays once before we enter into b and then the a' is the same as before.
So there you have it: composite ternary. A closed--simple binary?, B open (as it ended in c# minor the dominant of its starting f# minor) simple ternary, and A closed.
Everyone should listen to this, cause it's so pretty.