Beethoven Piano Sonata No. 14 I. Adagio sostenuto

The first movement, in C♯ minor, is written in modified sonata-allegro form. The movement opens with an octave in the left hand and a triplet figuration in the right. A melody that Hector Berlioz called a "lamentation",[citation needed] mostly by the left hand, is played against an accompanying ostinato triplet rhythm, simultaneously played by the right hand. The movement is played pianissimo or "very quietly", and the loudest it gets is piano or "quietly".

The adagio sostenuto has made a powerful impression on many listeners; for instance, Berlioz said of it that it "is one of those poems that human language does not know how to qualify". Beethoven's student Carl Czerny called it "a nocturnal scene, in which a mournful ghostly voice sounds from the distance". The movement was very popular in Beethoven's day, to the point of exasperating the composer himself, who remarked to Czerny, "Surely I've written better things".

In his book Beethoven's pianoforte sonatas the renowned pianist Edwin Fischer suggests that this movement of this sonata is based on Mozart's "Ah Soccorso! Son Tradito" of his opera Don Giovanni, which comes just after the Commendatore’s murder. He claims to have found, in the archives of Wiener Musikverein, a sketch in Beethoven's handwriting of a few lines of Mozart's music (which bears the same characteristic triplet figuration) transposed to C♯ minor, the key of the sonata. "In any case, there is no romantic moon-light in this movement: it is rather a solemn dirge", writes Fischer.