one 用作替代词主要用于替代“a＋单数可数名词”,表泛指；
若需特指,则用 the one.如：
A fast train is one that goes fast.快车是一种行驶快的火车.
Open the drawer on the left,the one with a key in it.打开左边的抽屉,上面有钥匙的那个.
I want very much to see these films,especially the one you mentioned.我很想看这些电影,特别是你提到的那一部.
注：the one与that均可表特指,但前者只用于替代单数可数名词,而后者则可用于替代可数或不可数名词.
This room is larger than that one.