how can i open any solidworks assembly file of 2010 version into solidworks 2009 ?

thank you all

maybe this can help
http://grabcad.com/questions/tutorial-how-to-recognize-parasolid-file-in-solidworks

Saving as a step file is the best, it saves each part out individually and the "assembly" so you can easily reassemble in an earlier SolidWorks version. However, all SolidWorks features and sketches will be lost.

As Hans said, solidworks have problem with future versions,but you can save as iges or step and after opening file in your desire version , use the features work.but may be your design tree would not be such beauty !!!