Complex sentence: Finally disgruntled by the Lorax, the Once-ler declared his intention to keep "biggering" his operations, but at that very moment, they heard the very last Truffula tree get cut down.
Split into simpler sentences: Finally disgruntled by the Lorax, the Once-ler declared his intention to keep "biggering" his operations. But at that very moment, they heard the very last Truffula tree get cut down.