After seeing '3 wrong' and reading your tip, I can understand how the vertices and the snapping tool work.For 3 wrong above I would wager you have somehow selected all four verts (2 on top and 2 on the bottom) and then moved all four. To avoid this kind of confusion I would recommend using the "merge vertex tool".
Thanks Bullet - I don't think I'll be using Maya for much room building...adding and deleting for rooms in Maya...it is not the best application for creating rooms....it can be achieved..however Sketchup is much better for this.