Hi how do u convert from dwg 3d file to part file in inventor. i am using the inventor 2010...

You don't need to convert anything. Just use the DWG Import Wizard to open your Autocad file in Inventor.

Inventor will do everything for you but DWG viewers also available
http://www.microconceptsretail.co.uk/design-review-46-c.asp

In AutoCAD: Export to a sat og iges file
In Inventor: Open - Saves As a ipt or iam
 
Best luck