Complex sentence: A German counter-bombardment swept the ground west of Ovillers and prevented the approach of French reserves and during the night the French survivors of the attack fell back to the French front line, except at La Boisselle.
Split into simpler sentences: A German counter-bombardment then swept the ground west of Ovillers and prevented the approach of French reserves. During the night the French survivors of the attack fell back to the French front line, except at La Boisselle.