Here is a new Blender 2.8 beginner tutorial for using the mirror modifier. I show how to mirror across the pivot point of the object and also how to use a mirror object in the mirror modifier tab.
I also explain the merge and clipping feature of the mirror modifier and then rotate the mirror object so that you can see the effect of the merge distance.
See my social profiles here
Production Music courtesy of Epidemic Sound: http://www.epidemicsound.com