Hi nukkle, thanks for testing MoI!

> A. the surface is very smoth and round, or curvy,,,frmo curve
> to curve. how can i make the surface get a hard line in it, where
> the yellow curves are

How was your surface created? Was it by using the Loft command?

There is an option in the Loft command for "Loft style", and if you switch it to Loft Style = Straight, you will get hard lines between each surface rather than a smooth surface.

If you want to maintain the general shape of that current surface but put a small sharp bit in it, then generally the way you would do that would be to draw in some planar curves in a side view so that they are arranged like this:




Then you would trim the surface using those planar curves as the cutting objects:






Now you can connect the trimmed edges with a Loft which will be straight, or also draw in a small profile curve and do a sweep with those trimmed edges as the rails.


> B.How can i make a surface bewteen B and C . B is one part
> of a whole curve that i cant, select, how to?

Select the curve and then use Edit/Trim to break it into smaller pieces - when it asks you to pick cutting objects in the Trim command, you can push the "Add trim points" button to pick some points on the curve to cut it with if you don't have any other cutting objects around to intersect it.

- Michael