Complex sentence: On 23 September, Crown Prince Rupprecht wrote in his diary that the higher ground at Zonnebeke and Gheluvelt was vulnerable to another British attack and that it was vital that the German counter-attack system did not fail, as it had on 20 September.
Split into simpler sentences: On 23 September, Crown Prince Rupprecht wrote in his diary that the higher ground at Zonnebeke and Gheluvelt was vulnerable to another British attack. It was vital that the German counter-attack system did not fail, as it had on 20 September.