Complex sentence: Methyl tert-butyl ether MTBE is a gasoline additive, used as an oxygenate and to raise the octane number, although its use has declined in the United States in response to environmental and health concerns.
Split into simpler sentences: Methyl tert-butyl ether MTBE is a gasoline additive is used as an oxygenate and to raise the octane number. Its use has declined in the United States in response to environmental and health concerns.