I'm very new to revit and I have gone through all the massmodeling tutorials that Autodesk provide but I'm still stuck. I want to create an object made predominately out of triangular shapes, I figured that adding edges to make the shapes and then move the nodes around would be the best way but its not working.

I am only able to create an edge between the original edges of the object not a new edge that I have made. The blue line shows where I want to make a new edge but can not. Is there away around this or is there a better approach? I hope my problem make sense and any help would be greatly appreciated.