If you have the 1.4 version of Scratch, open up the example project ScrollingDemo, and look at the scripts.
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It's easy. Just make a sprite called terrain and a variable called scrollx. then make a script so that the sprite terrain will change it's x coordinate by the variable scrollx. Then make a script so that when the left arrow is pressed, scrollx is set to 1, and if the right arrow is pressed, scrollx is set to -1, and if neither are pressed, it is set to 0.
Hope I helped!
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or if you want it slightly more complicated, make the terrain forever go to scrollx, make scrollx set to 0 at the start, and then if arrow keys pressed, scrollx is changed by 1 (left) or -1(right). This will help with the monster, you could make it forever go to scrllx + how far along you want it to be. Hope I helped! again.
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