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. volume transaction in yobit is good only pair altcoin
but you want trading bitcoin pair fiat money is good huobi btc-e
earn trading only buy in low price and sell in high price, if you want trading altcoin you can survey about coin, in coinmarketcap dot com