I need help with the mirror modifier. "how" is where I want it to copy to "why" is where it did

It's mirroring around your object's centre, not the cursor or world centre.

You can remove the modifier, move your object (in edit mode) to the other side of the line, then reapply the modifier. Or else set origin to 3D Cursor (after ensuring cursor is at the place you want to mirror around (like world centre).

If that's not the problem, I'm not sure what is.

I'm not sure I'm understanding what you're trying to do. You selected a mesh object, used the Mirror modifier, but instead of having it to the right of the original mesh object, which is how the Mirror modifier works, you want it to the left, and unattached? If that's the case, you should "duplicate" the original mesh, not "mirror" it.

Mirroring takes what's on the left side (let's say the top of a jacket" and creates it "mirrored" for use as the right side of the jacket. That way all you need to do is get the two pieces close together so you can weld the verts of the back seam together so it's one "single" piece, not two separate pieces.

Hope that helps some.

Miss B : Yes that helps a lot. Did not know that left to right was the standard option.

SilentWinter : Setting the origin to cursor did the trick. Thank you.

Ohhh, looks like SilentWinter and I pretty much cross posted, so I missed the comment about resetting the origin. Glad to see that worked for you.