Complex sentence: Attacks by Malta-based aircraft were made on the main Italian fleet without serious effect, although they disabled the heavy cruiser Italian cruiser "Trento" which was finished off by the submarine HMS "Umbra".
Split into simpler sentences: Attacks by Malta-based aircraft were made on the main Italian fleet. They disabled the heavy cruiser Italian cruiser "Trento" which was subsequently sunk by the submarine HMS "Umbra".