Section 3, Lesson 21. I couldn’t figure out why my door (doorframe?) wasn’t mirroring (or actually wasn’t mirroring in the same way as his). I finally realized that I didn’t set-up my corridor on the same axis as he did. So when he said to mirror it on the X axis I actually needed to mirror it on the Y axis (or otherwise rotate my corridor around 90 degrees before I added the cube. Anyway, if anyone else runs into a similar problem I hope this helps solve it.
You could use
APPLY rotation on the door to align it with the world.
But it can also be destructive if you don’t know exactly what’s happening.
Better is the use of EMPTIES and using the empty as a parent to the door.
You will learn.