Schubert's Sonatina Op. 137 is a charming work that showcases his characteristic lyrical gift and harmonic richness. It offers a blend of delicate melodies and flowing accompaniments, typical of his more intimate compositions.
Developing a warm, singing tone and sensitive phrasing for the lyrical melodies is crucial.
3 Sonatinas (Op.137) is rated Henle level 4 of 9 — intermediate repertoire, roughly ABRSM grades 5–6. In the RCM system it corresponds to about level 6.
You should be comfortable at Henle level 4 — meaning you can already play level 3–4 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of 3 Sonatinas (Op.137) lasts about 15 minutes.
3 Sonatinas (Op.137) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.