1 it 特指某一事物。
There is an apple tree at the back of the house. It was planted by my father. 屋后有一棵蘋果樹,是我父親種下的。
I enjoyed the party last night; it was very good. 昨晚的聚會很好,我玩得很高興。
2 one 表示泛指,指同類中的任一個;the one 表示特指的一個。
Have you any books on gardening? I want to borrow one . 你有關于園藝的書嗎?我想借一本。
The train was crowded so we decided to catch a later one . 這趟火車太擠了,我們決定乘下一趟。
There is a dictionary on the bookshelf—is that the one you want? 書架上有一本詞典——是不是你想要的那一本?