Motion Tweening
 

Step 1: Open Flash and size the work area    from Modify | Movie choosing 300 x 200 for this example. Use any color for the background. Select the square drawing tool, no line color and any fill color. On the stage, draw out a square, then choose the black arrow tool from the toolbar. Select the square and drag it outside to the left of the stage. While the square is still selected, hit the 'F8' key to convert it to a symbol.

Step 3: Select frame 20 on the timeline, then grab and move the square on the stage to the far right outside.

Step 4: Right click anywhere between frames 1-20 on the timeline and 'Create Motion Tween' from the selection box. Then, repeat anywhere between frames 20-40.

Step 5: Rewind and play your movie to see the box move back and forth.