Chain crunches when changing gears?

i borrowed a bike from my friend and i think i messed it up! it has 3 gears at the front and seven at the back (counting these from the shifters). when i shift onto 2 at the front and i'm pedalling it starts to make a loud crunching noise and gets stuck going round so i have to push down harder. After a while the chain seems to become really loose and i can't pedal until i turn the pedals backwards and the chain fixes its self- but it still crunches going round and is noisy. How do i fix this before i give the bike back please!!!!! ThanksChain crunches when changing gears?
A noisy chain is probably shifting too slowly up or down. You can try changing the cable tension by adjusting the barrel adjusters connected to the cable on the downtube or on the derailleurs (any adjuster that can be hand tightened/loosened). If the chain gets stuck and falls off, it's probably a result of improper chain tension on the front derailleur. If adjusting the barrel adjusters doesn't work, you can always bring it to a bike shop for them to look at it. They can give you a price quote for the repair. For adjusting/tuning the derailleur limit screws, it probably will be less than $20.Chain crunches when changing gears?
k you didnt break it, its just wear and tear ont he chain and the acutal gear cogs, if oil doesnt fix it, tell himt o get it replaced.

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