I'm currently learning the subject Schubert Impromptu and, as an aid to understanding and memorisation, I want to try and analyse the piece harmonically, away from the piano.

Does anyone know of an on-line resource which might help me do this? I would like to be able to identify the various chord structures in relation to the key signature (Gb major). I've tried to do this using my limited knowledge of Harmony, but it's heavy going!