Being a lyrical composer it's hardly surprising that Schubert's two greatest symphonic movements - those that make up his Eighth Symphony, the Unfinished - should contain very beautiful melodies set against the sort of accompaniments usually found in songs. More surprising perhaps for a composer known more for his 'sleepwalking' than his purposeful striding is the sheer power of Schubert's symphonism. This is at its strongest in the first movement's overwhelmingly dramatic development section - one of classical music's most exciting stretches.
The first movement, marked 'Allegro moderato', opens with a brooding theme played in unison by cellos and basses. (Is it wrong to hear in its bare presentation echoes of plainchant?) The theme that initially seems like the main subject then follows on oboe and clarinet over of an ostinato accompaniment on violins. A tiny rocking figure on a minor second infiltrates its way into the tune. This, on woodwinds, will magically prime the recapitulation. (The sound it makes, incidentally, must have inspired Dvorak's wonderful symphonic poem The Wood Dove.) The second subject has a syncopated accompaniment which will return in the development section minus its beautiful melody and later on in the recapitulation in the form of a canon. Both the bridge passages in the exposition become dramatic at their climaxes, foretelling what is to come - that development section, which begins by brooding on the opening theme (which turns out to have been the main theme all along), building it over tremolos to a mighty climax. Then a shuddering descending figure alternates with the second subject's accompaniment in an heroic tussle. The issue is brought to a head thrillingly in the following passage as brass and strings struggle and reach a tragic climax, at which point the movement hovers between major and minor. Further tonal vacillation leads us towards the recapitulation. Following the recapitulation, the coda returns to brood for one last time on that opening theme before a beautiful close whose spirit looks forward half a century to Dvorak.
The second and final movement, marked 'Andante', begins with the warm, romantic sound of horns. They play the movement's three-note 'motto'. We then hear the lovely main theme - or rather themes, as it is a fusion of melody and a counter-melody. Schubert sings it to us, gently. A march interrupts the song briefly. Unaccompanied violins leap up an octave ethereally taking us to the plaintive second subject - a tune sung at first by a clarinet and later by an oboe. The early passing march was a warming of storms to come, but the storm when it comes is a short-lived one. When it passes we are left in a sunlit landscape full of beautiful exchanges between first violins and basses, then between clarinet and horn. This is romantic nature music. It leads us straight into the recapitulation. The ethereal octave leap returns in the coda to transport us into a blissful goodbye whose mood is not unlike that of the close of Mendelssohn's Midsummer Night's Dream Overture.
The 'Unfinished' is one of the best-loved of all symphonies, and it's not hard to see why.
As to the question of why Schubert never finished the 'Unfinished', well, speculation has been rife now for nearly two centuries. The most poetic explanation is the 'Unfinished' couldn't be finished because it was already finished - in the sense of needing nothing more added, being a perfect whole in itself. Though that is certainly how it seems to us, I very much doubt that Schubert thought any such thing. I suspect his reason for leaving it unfinished is much more prosaic. He probably put it aside to finish some time in the future, waiting for the time when suitable ideas would inspire him to pick it up again. He didn't know that leaving a masterpiece half finished in a drawer for a few years was going to be a problem because he didn't know he was going to die so young. Well, that's my guess anyhow, for what it's worth.