Complex sentence: Sohler commanded her for her work ups with the 7th U-boat Flotilla, and she then became a front boat of 7th U-boat Flotilla and set out from Kiel on her first war patrol on 19 August 1939 in the North Sea, returning on 15 September.
Split into simpler sentences: Sohler commanded her during her working up with the 7th U-boat Flotilla, she then became a front boat with that flotilla. She set out from Kiel on her first war patrol on 19 August 1939 in the North Sea, returning on 15 September.