Can anyone help me how to know if the bitcoin price will decrease?
and how to trade bitcoins and earn? 
Because i invest bitcoins in yobit.net but i didn't know how to properly trade bitcoins. Well,the only working strategy for me is this:
If you think that the price will increase-buy btc.
If you think that the price will decrease-keep your btc and don`t sell,the price will go up sooner or later.
Good luck with trading OP.