Each letter is a key on the on-screen piano below. Lowercase letters are white keys, capital letters are black keys. Open the piano and press them in order — no sheet music needed.
G G A G C B · G G A G D C · G G G E C B A · F F E C D CHappy Birthday is the first tune most people actually want to play for someone, and it teaches two things Twinkle never does: a 3/4 waltz pulse and a two-note pick-up before the downbeat. The melody only spans one octave, so your hand barely moves — but hitting the pick-up in the right place is what makes it sound like the real song.
In C major: G G A G C B, G G A G D C, G G G E C B A, F F E C D C. The two opening Gs are a pick-up, so they come before the first strong beat.
Almost always because of the pick-up and the 3/4 time, not the notes. Count 'one-two-three' and start the first note on beat three.