When the Moon is the opposite side of the Earth to the Sun.
This is what causes the so-called "phases" of the moon: although fully half of the moon is lighted, we can only see part of the lighted part. A person on earth hardly ever sees all of the lighted side of the moon. When he does, he calls the sight a "full moon."
No, during a full moon, we see the entire lighted side of the moon, whereas during a new moon, the lighted side of the moon is facing away from Earth so we see no lighted side at all.
the part that you don't see is covered by the clouds, therefore you can't see it.
One half of the moon, the side that faces the Sun, is always lighted. But we usually see only part of the lighted side (phases) as the Moon orbits the Earth. When the Moon is directly between the Earth and the Sun (a new moon) none of the lighted side, facing the Sun, is visible from Earth.
quarter moon
It is called Waxing! _________________________ The Moon is always the same size. What we see increasing is the lighted part of the nearside of the Moon. When it is increasing, we call it "waxing"; when we see it decreasing, the word "waning" is used.
A2. The Moon keeps one side locked towards Earth. Thus whether it were illuminated or not, we can never see the far side of the Moon.Incidentally, there is a marvellous movie of the complete Lunar cycle under Moon/Tidal effects in Wikipedia.A1. The sun casts a light on one side of the moon and the other side is a dark shadow. So if there is a half moon, for example, from earth, we can see half of the lighted side and half of the dark side, which makes it look like the moon is cut in half. Since the Earth rotates, we can see a different side every night, and that's how we can see more or less of the lighted side of the moon. For a new moon, Earth can see the whole dark side and none of the lighted side. For a full moon, Earth can see the whole lighted side and none of the dark side.
You see just that part of the moon that the sun is shinning on.
The Moon has only one shape; it is round. Spherical, actually. The appearance of the Moon changes depending on where the Sun is, and how the light is shining on it. When you are able to see the whole lighted part of the Moon, it is "full", and when you can only see the dark part, the Moon is "new". But it is round all Month long.
because the light reflects on the moon and we only see the lighted side
The amount of lighted side of the moon you can see is the same during first quarter and third quarter phases. These phases occur when half of the moon's surface is illuminated, and they represent the halfway points between new moon and full moon phases.
Those shapes, and the corresponding phases, are called "crescents" ... when the lighted part that we see is less than half of a full disk.