One bar per line
Each line of the sheet is one bar: twelve triplet notes, four groups of three. A bracket at the start of a group adds what sounds with it, the bass note and, from bar 5 on, the melody note. Line one, [*W] - - T - - u, is the C♯ minor chord broken into three: G♯3, C♯4 and E4 over a C♯3 in the bass.
The harmony changes slowly, which is what gives the opening its calm: bar 2 over B, bar 3 over A and then F♯, bar 4 over G♯, back to C♯ minor in bar 5. Watch the capitals as they change: t in bar 4 is a plain C (Beethoven writes it as B♯), and Q and Y are F♯3 and D♯4.
The melody enters
At the end of bar 5 the right hand’s little finger adds the melody: a G♯4 (O) on the bar’s last beat, then a quick second G♯ just after the last triplet note, written uO. That dotted figure, long G♯ and a short one, comes back at the end of bar 6 (IO) and leads to the melody’s held notes in bars 7 and 8: G♯, A (p), G♯, F♯ (I), B (a), and at last E in the closing chord [0Wu].
On a computer keyboard the melody and the triplets are in the same hand, which the piano doesn’t mind: press a bracket’s keys together or one after another.
Practising on a phone
The chords here can be wide. [$ep], F♯2 under A4, spans more than two octaves, wider than the 25 keys a phone can show, so a phone’s keyboard shifts from the low F♯ up to the two As and waits for each one. On a computer screen all 61 keys fit. Beethoven marks the movement Adagio sostenuto, and it should never feel hurried.
What Play it for me leaves out
Play it for me is a guide to the order and rhythm of the notes, not a performance. It sounds each step only until the next one starts, so the bass notes and the long melody notes, which Beethoven holds for a whole bar or half a bar under the moving triplets, stop after one triplet note. Use it at Slow to hear where each note falls. When you play the passage yourself, hold Space as the sustain pedal and change it with each new bass note; then the bass and the melody ring on as written. The score asks for this too: its first bar carries the instruction “si deve suonare tutto questo pezzo delicatissimamente e senza sordini”, play the whole piece very delicately and without dampers.
About the sonata
The Piano Sonata No. 14 in C♯ minor, Op. 27 No. 2, dates from 1802 and was published the same year in Vienna by Cappi, with a dedication to Countess Giulietta Guicciardi. Beethoven titled it Sonata quasi una fantasia; the nickname “Moonlight” is not his, and IMSLP lists it as apocryphal. The first movement, Adagio sostenuto, runs 69 bars (IMSLP). The music has been in the public domain for well over a century.
Another Beethoven opening, lighter and quicker, is spelled out on the Für Elise page.