Hoi Pieter,

Uit Nederland neem ik aan? :)

As Burr stated NURBS modeling differs fundamentally from polygon subdivision modeling. NURBS modeling is usually about constructing surfaces using a structure of perfectly aligned / adjacent curves, or using Boolean operations to subtract, add or interesect solid NURBS primitives (contrary to polygon modeling, Booleans are very useful in NURBS modeling). Tweaking the control points of the resulting NURBS surfaces is something I usually avoid. In stead, I adjust the curves and recreate the patch from them if necessary (if MoI's History is active, then the surfaces will update in realtime when you adjust the source curves).

Check out the Construct section of the MoI reference manual for several different options for creating surfaces from curves and NURBS objects using Booleans:

http://moi3d.com/3.0/docs/moi_command_reference7.htm#Construct