You mean regular D.S. games? Good question! I myself asked this one before I bought my 3DS, and yes you can (I wouldn't of bought it if not). If you play a regular D.S. game then it automaticly turns off the 3D feature, but with 3D games you have the option to keep it on, or play the game 2D. Suprisingly enough, you can't play a 3D game on a regular D.S., as all 3D games have a special thing on the game cartrige that means it only fits in a 3.D.S.
Hope I've helped!
Hope I've helped!