Complex sentence: Time crystals extend the ordinary three-dimensional symmetry seen in crystals to include the fourth dimension of time; a time crystal spontaneously breaks the symmetry of time translation, which remarkably allows for the crystal to in perpetual motion.
Split into simpler sentences: Time crystals extend the ordinary three-dimensional symmetry seen in crystals to include the fourth dimension of time; a time crystal spontaneously breaks the symmetry of time translation. The crystal's pattern repeats not in space, but in time which remarkably allows for the crystal to be in perpetual motion.