use the right and left arrow keys to move the green paddle. pressing the arrow keys once moves the paddle a small amount. holding the arrow key down moves it faster, but there's a delay before it starts, so watch your timing. UPDATE: paddle control improved to remove/reduce delay.
based on the pong game instructions in the help file when you first start scratch. I added the background changes, the paddle controls, and the difficulty changes. UPDATE: Thanks to thepizzaman2 for improving the paddle controls *bow*