Complex sentence: He taught Musical Forms at the National Academy of Music from 1950-1955 as assistant-lecturer to Prof. Veselin Stoyanov, then specialised for a year at the Moscow Conservatoire under Aram Khachaturian.
Split into simpler sentences: He taught Musical Forms at the National Academy of Music from 1950-1955 as assistant-lecturer to Prof. Veselin Stoyanov. He then specialised for a year at the Moscow Conservatoire under Aram Khachaturian.