Think and discuss: based on what you have observed and learned, why do we see moon phases

First of all moon phases exist because the sun is on the other side of the earth when its night time, and the moon shines because it's reflecting the sun. And at different times the moon is in a different place in relation to the sun so we can only see part of it.
Also sometimes we can see the full moon slightly illuminated along with the current phase, and that's called earthshine. This is caused because the earth gives off light as well as the sun, so the light that the earth is giving of is reflecting off the moon. Earthshine isn't as bright as when the sun is reflecting off the moon because the sun is a lot brighter.

Good luck : )