Why does the moon seem to change its shape every night? The answer is the moon is a world in space, just as Earth is. Like Earth, it's always half illuminated by the sun. In other words, the round globe of the moon has a day side and a night side.
From our earthly vantage point, as the moon orbits around Earth, we see varying fractions of its day and night sides. Understand the various phases of the moon.