Complex sentence: Novae are thought to occur on the surface of a white dwarf in a binary system when they are sufficiently near to one another, allowing material (mostly hydrogen) to be pulled from the companion star's surface onto the white dwarf.
Split into simpler sentences: Novae are thought to occur on the surface of a white dwarf in a binary system. If the two stars of the system are sufficiently near to one another, material can be pulled from the companion star's surface onto the white dwarf.