A typical mountain bike has 3 chain rings in the front and 6 or more in the back. The shifter on your tight hand shifts the rear and the shifter on your left shifts the front. So let's start out easy put the chain on the middle chainring in the front. Now ride your bike. If you have shimano type shifters then the right shifter will shift the back gears. So as you ride along try shifting into an easier gear with your right thumb. Do you feel the difference? Now try shifting with your right forefinger. Can you feel it getting harder to pedal but your going faster? If so good. If that all worked great now try approaching a small not very steep hill. Before you get to the hill shift to an easier gear with your righ forefinger. Hopefully you cleared the hill. If so go to a flat area and try the same stuff with the left shifter. Good luck shifting your mountain bike if you still don't get it come back we will help you out some more.
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