Complex sentence: The Once-ler polluted the sky, river and landscape, until finally the last Truffula Tree fell outside, and the Once-ler realized what he had done while making the region uninhabitable with his business's pollution.
Split into simpler sentences: The Once-ler polluted the sky, river and landscape, until the last Truffula Tree fell. Finally, the Once-ler realized that he had made the region uninhabitable with his business's pollution.