Schubert's "Du bist die Ruh" is one of my favorite songs; beautiful lyrics, pretty piano part, and wonderful lines that seem simple enough but are actually very difficult to sing well. The opening 7 measures defies the stereotypical Schubert piano line with its light texture, both hands in the treble clef, and pianissimo marking. Schubert seems to be setting the scenery for the first line of poetry "You are rest, the mild peace" to the progression I, vi6, I6, I. The trick for the singer here is to have a whisper-like quality to their presentation- making sure to articulate every word, while maintaining a legato line and consistency of tone. The next 4 measures have almost identical chords with new text "You are longing, and what stills it." The difference is that this "longing" identified in the text is also in the music by prolonging the I6 harmony and moving on to a minor i in the next measure. Schubert fills this line with conflicting emotions to reflect the poetry "I consecrate you, full of pleasure and pain" by using the minor i, dominant, and secondary dominant functions. He resolves the poet's ambivalence in the next phrase though by placing a i, V, IV6, I64, V, I progression under "As a dwelling here, my eyes and heart." It's almost as if Schubert is trying to make the first "my eyes and heart" seem sacred after the i-V the listener has just heard by using the IV6 sonority. Singing this, I would try to make the first "my eyes and heart" sound like something very precious and then make the repetition a reassurance to the initial statement.
The piano interlude in measures 26-30, I interpret as just a prolongation of the tonic because of the constant E flat in the bass. Perhaps that E flat can be heard as the "peace" portrayed in the poetry. Obviously, there's a lot going on in the rest of the piano part, but there's a reason for that E flat and I think it's to establish the idea of peace through use of a constant tonic presence.
The next section is almost exactly the same as the first only to different text. There are a few differences such as an extra secondary dominant and a neighboring tone here and there, but mostly, it's just preparing for the upcoming section. Here's the translation just in case you don't have one handy: "Come live with me, and close quietly behind you the gates. Drive other pain out of this breast. May my heart be full with your pleasure." Notice Schubert has the same text painting scheme in the second verse. For instance, words like "pain" go on dominant harmonies while words like "pleasure" belong to tonic. Overall, the phrase, uses conflicting language- hence the dominant and secondary dominants.
Performing this section, I would have a slightly elevated mood in the first line "come live with me" as opposed to the opening of the song. Certainly I would articulate "drive other pain" with the conviction that a minor i deserves and glide it into the very next line smoothly until the tonic resolution which is reinstated by the following piano part.
This next section is often referred to as the most exciting part of the song and although I definitely appreciate all of its amazing qualities, I can't fail to recognize the fact that it wouldn't be so special without the preceding "whispery" pianissimo sections. Let's say Schubert just plowed right into this song with this section (like some composers choose to do, yuck!) with no gentle, soft part to come before,- then it wouldn't be as exciting and unabashedly romantic as it is. Ok, so this next part uses modal mixture to get the excitement going. Schubert goes from I-flatVI-flatIII-flatVI-V-I-IV, all in the matter of 7 measures. Placing this kind of progression under this text, I have to think that Schubert probably heard these words as being the most meaningful. Translation: "The tabernacle of my eyes by your radiance alone is illumined. Oh fill it completely!" But there's also a part of me that thinks perhaps he thought that they were not as important and that at this point in the song, the music and harmonies were to be the highlight. After all, it is naturally more difficult to hear text when being sung to chords such as flat sixes and flat threes. I also wonder about the IV chord before the measure of a rest. Was Schubert foreshadowing this climax earlier with his As? Is it just a coincidence that the singer's climactic note is an A? Why does Schubert put in that measure of rest before finishing the phrase with a quiet statement that would've felt at home in the first two sections of the piece? Probably to reinforce the mood of the song: rest, peace, calm. What could be more restful than silence? And the performers and the listener certainly need some after that long line of crazy- romantic excitement. The piano gets a soft statement in, like the singer before the singer lights up again with another climactic line, identical to the previous. This time, after the measure of literal rest, Schubert actually marks in the pianissimo directions for a final "whisper" from the singer that uses the sweet-sounding progression IV6, I64, V, I. But the singer ends on the fifth of the chord, meaning that the piano has to finish this resolution, making it even more romantic because the performers enact what the poetry wishes. Which is exactly why this is one of my favorite songs- you get all the romance of "The Notebook" in one 3 minute song. =)