Complex sentence: The quartet is named for the theme of the second movement, which Schubert took from a song he wrote in 1817 of the same title; but the theme of death is palpable in all four movements of the quartet.
Split into simpler sentences: The quartet is named for the theme of the second movement, which Schubert took from a song he wrote in 1817 of the same title. The theme of death is palpable in all four movements of the quartet.