Please cut ( 
the curved surface with the flat surface and publish the results.
Thank you.
test suddividi.3dm (3.0 MB)
does not split or trim, using Rhino 5.2.4
Hi Simon - did Rhino make this surface?
-Pascal
your splitting surface runs very accurately over the controle points so one way to split it
is to duplicate the surface turn points on and just remove one side of each surface by selecting the points
but its a pretty messy surface so however you created, maybe redo that before you make any further moves.
Hi @pascal
This area was built with Rhino more than a year ago. It is part of a polysurface. This surface in the final model is cut off and hundreds of unions and subtraction boolean. Having to draw a curve, I extracted a part of the cut surface, I canceled all cuts by restoring the original surface, but you can not cut it.
It was obtained by the network of curves command. It is a long time past, and as the file has had dozens of files, at the moment are not able to recover the original curves. The surface, with all its faults, is exactly what I wanted, as it corresponds to a real model, and can NOT be modified.
Simon
Hi Simon - use a ‘private message’ here on the forum. Click on my little robot guy to see this option…
-Pascal
Hi Simon - on your original surface with al the holes and trims : UntrimAll then split. I’m curious as to what process you used on the trimmed up surface to get to the bad one - do you recall what commands you used?
-Pascal
As I understand it, I wonder how I got the surface “bad” starting from the one I sent you. I used the command cancels the cut. I also tried using the “remove from the surface” edge.
Because he used the command “Untrim” instead of “UntrimAll” is a mystery.
Simon
Hi Simon - so with Untrim, you clicked on each edge and hole to untrim? I can only offer my sympathy - UntrimAll, UntrimHoles, UntrimBorder are good ones for the future though… I’ll try that one by one untrimming here and see if I can make the bad object.
So far no luck, just a lot of clicking.
-Pascal
Untrim on leading edges, then “remove surface edge” over the holes (with a unique selection)
Simon