I'm trying to understand AC2006 3D operations.  I want to draw a hollow box and cut a circle out of one face, so that the inside can be seen through the hole.  I drew a 3D box from the Surface toolbar.  I drew a circle on one face.  But since the circle is not a 3D object, the Subtract command doesn't work.  I thought of drawing a 2D solid, but there is no provision for circles.So I tried extruding the circle, then moving the resultant cylinder so it was centered on the face of the box.  Then I tried to subtract the cylinder from the box, but the program told me no solid objects were found.  I tried the same thing, but using a 3D solid cylinder.  Again, no solid objects were found.  !? wtf ?!I've considered the possibility that the faces of the box have no thickness.  Can this be the problem?I am used to DesignCad where I could convert a 2D circle into a solid and subtract it from another solid.  AutoCad doesn't seem to have that capability.  If someone knows the way to do this, I'd appreciate hearing from you.Thank You,Roger

AutoCAD (especially the newer versions) has a quite powerful solid modeller. So it is easier do to such operations with 3D solids (not 3D meshes as you did).
But you can still subtract 2D faces in AutoCAD - just make regions from them (explode the 3D mesh of the box and use the Region command to convert it to a region; make a region from the circle too).

O Boy!  I'm embarrassed.   I discovered what I'd been doing wrong.  I thought I had to click on the solid to be subtracted from, then the next click had to be on the solid to be subtracted.  Then I'd hit Enter.  But I discovered if I'm only dealing with two solids, the first click, on the solid to subtract from, must be followed by the Enter.  Then click on the solid to subtract and hit Enter again.  And the program behaves as advertised.So, my bad.Thanks for the learning experience.

Subtract should work as described in the Help (both on regions and solids) - it has two sets of "Select objects" prompts. The first is for the source objects, the second for the subtracted objects.