Complex sentence: During a gun duel, the commander of the "Galileo Galilei" was killed, and the submarine was captured by the armed trawler and used by the British as HMS "X2".
Split into simpler sentences: During a gun duel, the commander of the "Galileo Galilei" was killed, and the submarine was then captured by the armed trawler. The submarine was subsequently used by the British as HMS "X2".