bike chain keeps falling off. i know how to put it on. it will stay on if i turn the pedal backward, but when i try to make it pedal forward (to make the wheels move) it will move the wheel a little bit but then the chain will slide off the chain. i know that the chain isnt too greasy because i cleaned it off to see if that was the problem. it kinda looks like the gear that turns the chain is out of line or something. but i dont know that much about bikes so please help me. it will be very appreciated =]