I'm new to the program and seem to constantly run into issues with settings blocking different commands. For example, I am trying to extrude along a path. I draw a nurbs curve and a circle on the page. The tab for the command does not highlight. Extrude highlights but says objects are illegal. Is there a cheat sheet that give needed settings for each command. I'v tried screen plane, layer plan, 2d, isometric. What are the proper settings for this command? Thanks.