when i used mirror, why the moved edge did not used mirror?

[jxsb6]

when i used mirror, why the moved edge did not used mirror?

see my file and video 

[Christoph_Schaedl]

Mirror is a one time operation. Not a live operator.
You need to instance the object by -1.

Or dont cut the object in half and use the symmetry option from the top.

[jxsb6]

1.You need to instance the object by -1.

how to instance?

please explain with tutorial video , because i am a beginner

2.

use the symmetry option from the top

how to use the symmetry option from the top?

please explain with tutorial video , because i am a beginner

[mcginnc]

I think for the 1st answer, you can (also) mirror an object using the Edit > Duplicate Special. Use -1 to mirror it around the z - axis. See Duplicate_Instance.mp4. For the 2nd option, if you turn the Cut Geometry in the Mirror options OFF, you can drop down and choose Instance. That way when you move an edge, the corresponding one will move also. See Mirror_Instance.mp4. Hope this helps.