Complex sentence: On 21 October, attacks by the 4th Army reserve corps were repulsed in a costly battle and on German attacks were conducted to the north, on the Yser by the 4th Army and to the south by the 6th Army.
Split into simpler sentences: On 21 October, attacks, by the 4th Army reserve corps were repulsed in a costly battle. And on German attacks were conducted to the north, on the Yser by the 4th Army and to the south by the 6th Army.