Why don't you see the image of your face when you stand in front of clear glass window pane?

Any translucent surface both reflects and refracts light. By refraction, I mean that it bends the light a bit, but lets it through to the other side. Now, reflection for such surfaces is much less than refraction (unless there's total internal reflection, but that’s irrelevant for glass + air).

During the day, you have light from your room being largely refracted out, and reflected back inwards a tiny bit. The outside light does something similar. It is largely refracted into your room, and reflected back outside a tiny bit. So, the majority of the light you see coming from the window is due to the outside light. You will see a reflection if you look carefully (exactly how carefully depends upon the lighting of your room)

Now, during the night, there is little or no light coming from the outside. So the majority/all of the light you see is due to reflection. So you see the reflected image.

Light travels in a straight line" this is known as the rectilinear propagation of light. An image is formed whenever an object comes in the way of light. Since the glass window pane is transparent, light simply passes it and hence no image is formed