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. Usualy many traders using method analysis technical for making prediction of price will going up or going down and some traders using method analysis fundamental for making prediction of price, so depend on your strategy be used on trade on yobit.net because i don't know about your experience in trading. I think you must learn about analysis technical first and you can searching on Google.