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.
The accident was similar to one that happened in 2008. 这个事故与发生在2008年的事故类似.
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.
Here are six rings. Pick out the one you like best. 这里有六枚戒指,选出你最喜欢的一枚.
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.
Here are six rings. Pick out the one you like best. 这里有六枚戒指,选出你最喜欢的一枚.
另外附上：
代词that, one, the one, it的用法区别
① that是特指,代替前面提到的不可数名词或单数可数名词.代指前面提及的同类而非同一物,其复数用those,后面一般有后置定语.
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,指代前面提到的同类人或物（可数名词）中的一个,其复数用ones,前面一般有定语.
I bought five pencils for my children, two red ones and three green ones.
③ the one 是特指,指代前面提到的同类人或物（可数名词）中的一个,其复数用the ones.
This picture is the one that I drew yesterday.
④ it是特指,代替前面提到的那个特定的事物.
I lost my dictionary yesterday. I haven't find it so far.
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