Suppose you could see the sun and stars at the same time. suppose you see that the sun is in the constellation taurus during the morning hours when the sun is in the eastern part of the sky. eight hours later, you see that the sun in the western part of the sky, but it is still in taurus. this is because

The Earth rotation

Explanation:

The Earth rotates in its own axes, that is the reason why it can be observed the  Sun and some other starts with the naked area. And apparently all the sky rotates, but actually   is the planet the one that is constantly moving.

If the sun is close to Earth, so the light hours during the day are longer, and the visible of sun and stars is no so common.

When the sun is apart to the Earth, so the day hours are smaller, and the probability to observe the sun and the stars at the same time increases.