I decide which one to buy.为什么要用which one to buy？而不用which to buy？谢谢。

如果用which to buy 的话，句子意思就变成了 谁去买，而which one to buy就是指要买哪一个，which是疑问词，to 是介词，中间一定要有名词！！！

不知对不对哦~~昰照我平时的理解而已！！！
呵呵