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.
if we talk about yobit trade i dont think this analyze is needed because most of coins inside yobit is just pnd so it is really hard to predict how any coins will move the only thing that i can say is avoid those coin that consistently broadcast inside the chat and those coin that have big volume if you have time kindly look here inside the forum and fins quality coin and review all the potentials before investing/supporting it.
Well what was you said it was analysis of yobit, so on yobit still be needed analysis on there although with different method. But in my opinion it will become same method if we make analysis technical on yobit and you just use analysis fundamental.