one 用作替代詞主要用於替代“a＋單數可數名詞”，表泛指；若需特指，則用 the one。如：

A fast train is one that goes fast. 快車是一種行駛快的火車。

The accident was similar to one that happened in 2008. 這個事故與發生在2008年的事故類似。

Open the drawer on the left, the one with a key in it. 開啟左邊的抽屜，上面有鑰匙的那個。

Here are six rings. Pick out the one you like best. 這裡有六枚戒指，選出你最喜歡的一枚。

I want very much to see these films, especially the one you mentioned. 我很想看這些電影，特別是你提到的那一部。

注：the one與that均可表特指，但前者只用於替代單數可數名詞，而後者則可用於替代可數或不可數名詞。

Here are six rings. Pick out the one you like best. 這裡有六枚戒指，選出你最喜歡的一枚。

另外附上：

代詞that, one, the one, it的用法區別

① that是特指，代替前面提到的不可數名詞或單數可數名詞。代指前面提及的同類而非同一物，其複數用those，後面一般有後置定語。

The weather in Kunming is better than that in Wuhan.

The population of China is larger than that of any other country in the world.

② one是泛指，指代前面提到的同類人或物（可數名詞）中的一個，其複數用ones，前面一般有定語。

I bought five pencils for my children, two red ones and three green ones.

This picture is the one that I drew yesterday.

④ it是特指，代替前面提到的那個特定的事物。

I lost my dictionary yesterday. I haven"t find it so far.