Complex sentence: It was not until 1937 that Rubbra's first symphony was completed and, having composed this, symphonies 2,3 and 4 followed in quick succession, the fourth being completed in March 1942.
Split into simpler sentences: It was not until 1937 that Rubbra's 1st symphony was completed. Symphonies 2,3 and 4 followed in quick succession, the 4th being completed in March 1942.