Complex sentence: The ones who land in 1977 reunite with the other survivors who have lived for three years with the Dharma Initiative and attempt to change past events in order to prevent the Oceanic plane from crashing in the future.
Split into simpler sentences: The ones who land in 1977 reunite with the other survivors who have lived for three years with the Dharma Initiative. They then attempt to change past events in order to prevent their Oceanic plane from crashing in the future.