Complex sentence: The zero-energy universe states that the amount of energy in the universe minus the amount of gravity is exactly zero; the only kind of universe that could come from nothing, assuming such a zero-energy universe "is", already, nothing.
Split into simpler sentences: The zero-energy universe states that the amount of energy in the universe minus the amount of gravity is exactly zero. That is the only kind of universe that could come from nothing, assuming such a zero-energy universe "is", already, nothing.