Complex sentence: The lunar phases change cyclically as the Moon orbits the Earth, according to the changing relative positions of the Earth, Moon, and Sun. The moon and the Earth are tidally locked, and therefore the same lunar surface always faces Earth.
Split into simpler sentences: The lunar phases change cyclically as the Moon orbits the Earth, according to the changing positions of the Moon and Sun relative to the Earth. The Moon and the Earth are tidally locked, therefore the same lunar surface always faces Earth.