We know when it is daytime because it is light. Daylight comes from the Sun.
Things which make light are called sources of light.
The Sun is a bright source of light. A light bulb is not as bright as the Sun.
Candles are not a bright source of light, we say they are dim.
We can see because light enters (goes into) our eyes. Very bright lights, like the Sun can hurt our eyes.
When there is no light (when it is dark) we cannot see. We use lights in the dark so that we can still see things.
The Moon is not a source of light. The Moon reflects light from the Sun.
OK, so let's see if that makes sense!