不是。。。就是If it's not one ,it's another one.There's a possibility.They all have a 50%(But there are exceptions in different contexts,exp:既然不是A，那一定就是B了。ji ran bu shi A na yi ding jiu shi B le。If it's not A,it must be B.The last possibility has been eliminated,so it must be another one.)不是。。而是Not this one ,but that one.Indicating that the former has been denied.The latter should be correct.There is no possibility.It means that the person who said it thinks the latter is right.exp：我要的不是沙拉酱，而是番茄酱。wo yao de bu shi sha la jiang er shi fan qei jiang。What I want is not salad dressing but ketchup. My explanation is not professional enough ,but I hope it can hope you.

In Chinese, we are used to putting the evaluation of a thing behind.exp:我的衣服质量特别好。The quality of my clothes is very good.I addition to describing things ,we can also say so in other ways.exp:他跑得很快。ta pao de hen kuai。He runs very fast.他人不错。ta ren bu cuo.He's a nice guy. My explanation is not very professional.I hop it is helpful to you.