Look at the ray diagram below, the red stick is the object, and the blue one is its image. Change the position of the object, and see what happens. But don't forget to place the flat mirror to the place where two light rays crossed, so that you can see the image
And there's one thing to remember, if the image you see is upside-down, we call this a "real image" of the object, if it's not, we call that a "virtual image" of the object. A real image, can be seen only seen by a flat mirror which faces the curved mirror we use, but we can see a virtual image in the curved mirror directly.
Even though in real life we have to see a real image by a flat mirror, but usually we don't draw the mirror in a real diagram.
You might have noticed:
1. When the position of the object is exactly same as the "F", the two light rays are parallel to each other, which means they cannot cross at each other to form an image.
2.When the position of the object is exactly at the "C", the real image of the object is exactly the same as the object itself.
You might find out from the ray diagram that there's a relationship between object's distance(v) and its image's size and distance(u) to the concave mirror.
For the relationship between "v" and "u", we have this formula (mirror formula) below:
We could come up with this formula through some simple trigonometry.
And for the image's size, we have this formula (Magnification formula):
Technics terms: