Complex sentence: Jane Joseph (1894--1929) was an English composer, arranger and teacher of music, whose potential was first noticed by the composer Gustav Holst when he taught her composition at St Paul's Girls' School.
Split into simpler sentences: Jane Joseph (1894--1929) was an English composer, arranger and teacher of music. Her potential was spotted by the composer Gustav Holst when he taught her composition at St Paul's Girls' School.